FLORIDA STATE MUSEUM
THE CRANIAL ANATOMY OF THE HOG-NOSED
W. G. Weaver, Jr.
Rrb7 SAhe M-m-
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Published June 9, 1965
THE CRANIAL ANATOMY OF THE HOG-NOSED SNAKES
W. G. Weaver, Jr.1
SYNOPSIS: The cranial osteology and myology of the Xenodontine snake genus
Heterodon are described and correlated with certain aspects of the trunk muscula-
ture. Comparisons are made with the genus Xenodon and the viperidae.
Ileterodon, and to a lesser extent Xenodon, are similar to the Viperidae in
many features of their cranial and trunk myology.
A Xenodontine protoviper is hypothesized that gave rise to three present-
day snake groups: (1) the advanced xenodontine snakes such as Xenodon, (2)
the more primitive but specialized Heterodon, and (3) the vipers.
TABLE OF CONTENTS
Introduction ....-..... 276
Materials ........... 276
Systematic Position of Hetero-
don and Xenodon 276
Distribution of Heterodon
and Xenodon ---.-------- 277
Cranial Osteology ....--. 278
The Cranial Unit ---..--..-- 278
The Nasal Unit -- ...-... 282
The Palato-maxillary Unit 283
The Mandibular Unit .... 286
The Hyoid Unit -....------------- 288
The Vertebral Unit .. .--
The Adductores Mandibulae
The Constrictor Dorsalis -..
The Intermandibular Muscles-
The Hyoid Muscles -...-- -
The Clands and Ligaments
Functional Aspects ......-
1 W. G. Weaver, Jr. is a graduate assistant at the Florida State Museum. This
paper is a thesis presented to the graduate council of the University of Florida in
partial fulfillment of the requirements for the degree of Master of Science. Man-
uscript submitted 28 Jan. 1965.
Weaver, W. C., Jr. 1965. The cranial anatomy of the hog-nosed snakes (Heter-
odon). Bull. Florida State Mus., vol. 9, no. 7, pp. 275-304.
In a study of the lateral jaw musculature of Causus, Ilaas (1952)
proposed that the solenoglyphs arose from aglyphous colubrids. An-
thony (1955) reached a similar conclusion from his study of fang
position. The primitive trunk musculature of the viperids indicates
that an investigation of the trunk and cranial morphology of the prim-
itive colubrids is in order. This paper describes briefly the cranial
ostcology and myology of a specialized primitive colubrid, Hetero-
don, and compares them with those of Xenodon, which some authors
consider close to Heterodon. The cranial anatomy of Heterodon has
never been described in detail, though Kellicott (1898) used H. platy-
rhinos as a general model for snake dissection. Haas (1931) and An-
thony and Serra (1951) described the cranial anatomy of Xenodon
The myological descriptions are based on dissections of 25 Hetero-
don platyrhinos, 12 H. simus, 6 H. nasicus, 3 Xenodon rabdocephalus,
and 1 X. severus. The osteological descriptions are based on skele-
tons of 11 H. platyrhinos, 7 H. simus, 2 H. nasicus, 1 X. merrimi, and
2 X. rabdocephalus. In addition I dissected specimens of Agkistro-
don, Bothrops, Crotalus, Trimeresurus, and Vipera among the Viperi-
dae, Constrictor and Eunectes of the Boidae, and Coluber, Elaphe,
Lampropeltis, Natrix, and Thamnophis of the Colubridae. I have
conformed where possible to the myological nomenclature outlined
by Kochva (1962).
SYSTEMATIC POSITION OF Heterodon AND Xenodon
The poor fossil record of colubrid snakes forces greater reliance
on a horizontal classification. Cope (1900) organized the subfamilies
of the Colubridae on the basis of hemipenial structure and the pres-
ence or absence of vertebral hypapophyses. He placed Heterodon
in the Dromicinae, a group he reserved for aglyphous colubrids hav-
ing a calyculate hemipenis with a double sulcus. Xenodon and
Lystrophis, genera he considered superficially similar to Heterodon,
he placed in the Xenodontinae, aglyphous forms in which the hemi-
penis has a double sulcus and an apical disk.
Dunn (1928), using Cope's characters plus certain features of the
dentary bone, grouped Heterodon, Xenodon, Lystrophis, and most of
the South American colubrids into the Ophiinae (Cope's Xenodonti-
nae). He defined the Ophiinae as South American (mostly) aglyphous
1965 CRANIAL ANATOMY OF THE HOC-NOSED SNAKES 277
or opisthoglyphous colubrids with or without vertebral hypopophyses,
and hemipenes with basal spines and distal calyces or apical disks.
Bogert (1940) found Dunn's definition of the subfamilies Natrici-
nae and Ophiinae inadequate for Old World genera. Schmidt (1949)
and Romer (1956) concurred in not recognizing the two subfamilies.
Smith (1964) proposed resurrecting the Heterodontinae, a name pro-
posed by Bonaparte in 1845 but not widely accepted. It includes
Heterodon, Xenodon, Lystrophis, and perhaps Lioheterodon and
others. The characters of the Heterodontinae, as redefined by Smith
(1964; 291), are: ". . bufophagous colubrid snakes having greatly
enlarged adrenals, a viperid type trunk musculature, a viperid type
jaw musculature, and a pro-viperid or viperid type of cranial kinesis."
Rossman and Wilson (1965) objected to the name Heterodontinae be-
cause of the nomenclatural complexities and inconsistencies its formal
adoption would involve. Smith concurred and withdrew his pro-
Little has been published about the relationships of the species
within the genus Heterodon, which Auffcnberg (1963) split into two
groups on the basis of vertebral characters, a platyrhinos group and a
nasicus-simus group. The Pliocene forms, H. brevis (Auffenberg,
1963) and H. plionasicus (Peters, 1953), are considered ancestral to
H. platyrhinos and H. nasicus, respectively.
DISTRBTImrN OF Heterodon AND Xenodon
The earliest fossil records for Heterodon are from the Pliocene of
Florida and Kansas (Auffenberg, 1963; Peters, 1953). The geographic
range of the genus is completely within North America east of the
Rocky Mountain chain (Edgren, 1952). Pleistocene records are all
within the ranges of the three extant species. The largest of these,
Helerodon platyrhinos, is a forest species of the eastern United States,
restricted to river bottom forests in the central prairie regions and
extending as far as southwestern South Dakota in ecologically suit-
able localities. H. simus, the smallest species, is almost entirely con-
fined to more xeric situations throughout the southeastern coastal
plain. H. nasicus, intermediate in size, is a snake of sandy areas
within the central plains of Canada and south to Tamaulipas and San
Luis Potosi, Mexico.
How many species comprise the genus Xenodon is not yet clear.
Most of them occur in South and Central America, and only one ex-
tends northward into Mexico (Smith and Taylor, 1945).
BULLETIN FLORIDA STATE MUSEUM
The skull is considered in terms of the six functional units set forth
by Albright and Nelson (1959): Cranial, nasal, palato-maxillary, man-
dibular, hyoid, and vertebral.
THE CRANIAL UNIT
The cranium consists of 2 frontals, 2 pre- and 2 postfrontals, an
occipital complex, 1 sphenoid, 2 pro-otics and 2 columellae auris.
The pre- and postfrontals and columellae are slightly movable.
FRONTALS. The frontals (fig. 1) are rectangular in shape dorsally, and
separated anteriorly by a grooved vertical septum. The lateral walls
converge ventrally to the narrow anterior projection of the sphenoid.
The dorsomedial surface is raised in a longitudinal swelling which
expands anterolaterally. The length of the bone in Heterodon nasi-
cus is shorter in relation to its width than the same element in H.
platyrhinos. H. simus is intermediate in this respect. In Xenodon
the frontals have a wide medial posterior extension which fits into a
notch in the parietal. The frontoparietal joint in Heterodon is nearly
PRERHONTALS. The prefrontals articulate dorsally with the frontals
and ventrally with the dorsal surface of the anteromedial process of
the maxillary and the lateral process of the palatine. These latter
two connections are flexible. The bone is wider ventrally than dor-
sally and is perforated by a large rostrally-directed fenestra in the
POSTFRONTALS. The postfrontals are slender, crescent-shaped bones
attached dorsally to the anterior edge of the lateral horizontal process
of the parietal and by ligaments to the maxillary and prefrontal bones.
The prc- and postfrontals of both genera are similar (fig. 2).
PARIETAL. The parietal is the largest of the skull bones. Its dorsal
surface is shaped like an unequal hexagon (fig. 1). The lateral bor-
ders expand posterolaterally into a horizontal lateral process which
contains a groove for the postfrontal on its anterior edge. The pos-
terior lateral wall is expanded as a vertical lateral process ventral
to the horizontal lateral process. A vertical wall that slants ventro-
rostrally forms the posterior border of both these processes and pro-
vides an origin for the adductor externus medialis, adductor externus
profundus and the elevator pterygoidei. The posterolateral border
joins the pro-otic and the posteroventral surface joins the sphenoid.
The parietal of Xenodon has a pair of dorsolateral grooves that start
1965 CRANIAL ANATOMY OF THE IIOG-NOSED SNAKES 279
Figure 1. Dorsal view of skull. A, Helerodon simus; B, II. nasicus; C,
H. platyrhinos; D, Xenodon merremi; E, X. rabdocephalus; Abbreviations: eo,
exoccipital; f, frontal; n, nasal; p, parietal; pm, premaxillary; pr, pro-otic; sm,
septomaxillary; so, supraoccipital; v, vomer.
BULLETIN FLORIDA STATE MUSEUM
from the postfrontals and converge posteriorly. These grooves fur-
nish an origin for the muscles listed above. In Heterodon the grooves
are shortened into the steep ventrorostrally-inclined walls.
Figure 2. Lateral view of skull. A, Heterodon platyrhinos; B, H. simus;
C, H. nasicus; D, Xenodon rabdocephalus; Abbreviations: af, anterior pro-otic
foramen; bo, basioccipital; eo, exoccipital; f, frontal; fo, foramen ovale; n, nasal;
of, orbital foramen; p, parietal; pf, prefrontal; pm, premaxillary; pr, pro-otic; psf,
postfrontal; s, sphenoid; sm. soptomaxillary; so, suproccipital, v, vomer.
The large orbital foramen in the lateral parietal wall is bilobed in
all the species of Xenodon examined and in Heterodon platyrhinos.
The dorsal lobe of the foramen is narrowed in H. sinus and absent in
H. nasicus (fig. 2). It is bounded anteriorly by the lateral parietal
wall and ventrally by the sphenoid. H. simus and H. nasicus the
parietal is relatively wider than in H. platyrhinos and its posterior
dorsal surface swells dorsally to meet the supraoccipital; this surface
is flattened in H. platyrhinos.
PRO-OTnc. The pro-otic is pentagonal; it connects with the parietal,
the supra- and exoccipitals, and houses the base of the long, slender
columella auris. The columella arises from a footplate within the
foramen and extends posteriorly, roughly parallel and ventral to the
squamosal. The anterior pro-otic foramen is entirely confined to the
pro-otic bone. A lateral ridge dorsal to both foramina proceeds from
the ventral anterior border posterodorsally to the end of the bone.
The foramen ovale is larger in Xenodon than in Heterodon. In Xeno-
don the anterior foramen is associated with two or three smaller
foramina which appear to be incorporated in the anterior pro-otic for-
amen in Heterodon.
OCCIPTTAIS. Four occipital bones comprise the rear of the brain case.
In Heterodon most of the occipital condyle is on the basioccipital; in
1965 CRANIAL ANATOMY OF TIE HOG-NOSED SNAKES 281
Xenodon it is mostly on the exoccipitals. All the occipital bones in
Heterodon border on the foramen magnum except in H. platyrhinos,
in which the supraoccipital sometimes does not. In Xenodon the
supraoccipital is much smaller than in Ileterodon and does not in-
trude into the dorsal border of the foramen magnum. As McDowell
and Bogert (1954) note, supraoccipital participation in the border
of the foramen magnum is rare in snakes; it has been reported else-
where only in Phyllorhynchus and Dasypeltis; in the Tropical Ameri-
can Boinae the supraoccipital overlaps the exoccipitals. The sagittal
crest is more prominent in Xenodon than in Heterodon.
SPHENom. The sphenoid (fig. 3) is an arrow-shaped bone that forms
the floor of the braincase. Its anterior portion extends rostrally from
its wide posterior end to the frontals as a tapering finger of bone.
Figure 3. Ventral view of skull. A, Ileterodon platyrhinos; B, Xenodon
merremi. Abbreviations: bo, basioccipital; eo, exoccipital; f, frontal; j, septo-
maxillary-frontal joint; p, parietal; pf, prefrontal; pin, premaxillary; pr, pro-otic:
s, sphenoid; sin, septomaxillary; v, vomer; vf, ventral vomerine foramen.
BULLETIN FLORIDA STATE MUSEUM
The broad posterior end of the bone furnishes an origin for most of
the constrictor dorsalis complex. On each side near the posterior end
is a dorsally concave ventrolateral process. A large groove from the
pro-otic extends anteroventrally to the sphenoid for insertion of the
retractor pterygoidei. The posterior midline of the sphenoid in both
genera tends to project ventrally. The groove from the pro-otic is
absent in Xenodon, being blocked by an elevated flange along the
sphenoid-pro-otic and the sphenoid-basioccipital borders. The bone's
general features are more similar to one another in Heterodon and
Xenodon than either are to other genera examined.
THE NASAL UNIT
The nasal unit consists of a premaxillary, a nasal, two septomaxil-
laries, and two voters.
PREMAXILLARY. In Heterodon the premaxillary is flattened antero-
posteriorly and consists of a vertical median bar arising from a ven-
trally directed horizontal bar. On each side and posterior to the ver-
tical bar a dorsolateral expansion gives the bone a characteristic flared
appearance (fig. 4). In side view the premaxillary is severely inclined
forward in H. nasicus, less in H. simus, and is nearly vertical in H.
platyrhinos (fig. 2). In H. nasicus the dorsal edge of the premaxillary
and the anterior edge of the nasal bone show a greater degree of
interdigitation than in either H. simus or H. platyrhinos. The distal
borders of the dorsolateral expansions are crenate in H. platyrhinos
(fig. 4) and smooth in the other two species. Both H. nasicus and
H. simus show depressions on the medial anterior face of the premax-
illary. These depressions are most extensive in H. platyrhinos. The
dorsal edge of the premaxillary is thicker in H. simus and H. nasicus
than in H. platyrhinos.
A B C
Figure 4. Anterior view of the premaxillary of Ileterodon. A, H. platy-
rhinos, B, H. nasicus; C, II. simus; Abbreviations: dl, dorsolateral expansion; hb,
horizontal bar; n, nasal.
1965 CRANIAL ANATOMY OF THE HOG-NOSED SNAKES 283
The premaxillary of Xenodon is generalized and resembles that
of Thamnophis, Elaphe, or Coluber. In Heterodon the premaxillary
attaches to the ventral surface of the septomaxillary by a broad, thin
process. In Xenodon the process is narrow and attaches to the medio-
ventral surface of the septomaxillary.
NASALS. The nasals are wedge-shaped in cross-section, medially
fused, and expanded posteriorly and anteriorly. The posterior ex-
pansion, particularly noticeable in H. nasicus (fig. 1), is so placed it
restricts vertical rotation of the nasal unit about the frontals. The
nasals of Xenodon are neither elongate nor posteriorly expanded and
are widest in the middle (fig. 1).
SEPTOMAXILLARIES. The septomaxillaries lie parallel and ventral to
each side of the nasal. Slender anteriorly, they expand posteriorly
into a lateral process curved dorsally at its tip. A posterior process
acts as an articular projection fitting into the anterior ventro-medial
border of the frontal. Thus the two septomaxillaries form a two-
pronged point of dorsoventral rotation for the nasal unit (fig. 3). In
Xenodon the septomaxillary does not cover the entire dorsal surface
of the vomer.
VOMEas. Posterior and ventral to the septomaxillaries are the vo-
mers. A bulb-like expansion occurs on the lateroposterior border
of each. Projecting from its medioposterior border each vomer has
a thin vertical process that is pierced ventrally by a foramen. The
dorsal edge of the vertical process is the insertion of the retractor vo-
meris. In Xenodon the bulb-like expansion projects dorsally, form-
ing a conspicuous bulge that is completed posteriorly by the posterior
portion of the septomaxillary. Both genera have a foramen on the
ventral surface of the vomer. The opening is rounded in Heterodon
and crescent-shaped in Xenodon. Medial to each foramen a thin, lon-
gitudinal ridge extends posteriorly to take part in the posterior verti-
cal process. The anteromedial border of the ventral vomerine fora-
men has a rostrally directed ridge medially delimiting the anteroven-
tral border of the bone.
THE PALATO-MAXILLARY UNIT
The palatine, pterygoid, maxillary, and cctopterygoid function as a
unit on each side of the head.
MAXILLARIES. The maxillary forms the anterior lateral border of the
chain of associated elements. It has two ventrally-directed medial
processes (fig. 5). The posterior process is on the medioventral sur-
BULLETIN FLORIDA STATE MUSEUM
face of the bone, anterior to the enlarged teeth. The enlarged ante-
rior tooth arises from a socket on the posterior border of this process.
The dorsal surface of the process is connected, via a loose band of
connective tissue, to the medial arm of the ectopterygoid fork. Con-
trary to Ditmar's report (1912), I found no grooves in the two en-
larged caudal teeth. The maxillary is shorter relative to basioccipital
length in H. nasicus and H. simus than in H. platyrhinos, and in
Xenodon than in Heterodon.
Figure 5. Ectopterygoid and maxillary bones of IIeterodon and Xenodon.
Left, dorsal view of right ectopterygoid. Right, medial and lateral view of max-
illary. A, Xenodon rabdocephalus; B, Xenodon merremi; C, Heterodon simus;
D, Heterodon nasicus; E, Ileterodon platyrhiinos; F, Heterodon platyrhinos (Medial
view); G, Xenodon merremi (Medial view); H, Heterodon platyrhinos (lateral
view); I, Xenodon merremi (lateral view); Abbreviations: a, anterior medial proc-
ess; b, posterior medial process.
PTERYGOIDS. The laterally flattened pterygoid (fig. 6) is bent laterally
at a point near the posterior end of the tooth row. Dorsal to the mid-
dle of the tooth row the bone gives off a lateral process which is
widest anteriorly and in H. platyrhinos is furnished with an anteriorly
directed finger of bone. On the dorsal surface of the lateral process
the same species has a wide groove which receives the ectopterygoid.
Both these latter features are absent from the pterygoids of the other
two species of Helerodon. The angle of the lateral bend is greater
in H. nasicus and H. simus than in H. platyrhinos. The pterygoid
1965 CRANIAL ANATOMY OF THE HOC-NOSED SNAKES 285
tooth row is relatively longer in Xenodon than in Heterodon, and the
posteroventral side is deeply grooved as opposed to the flattened pos-
terior end in Heterodon. The anterior bony finger of the lateral ptery-
goid process in H. platyrhinos is missing in Xenodon. In addition
the lateral process arises closer to the anterior end of the tooth row
Figure 6. Pterygoid bone of Heterodon and Xenodon. Left, ventrolateral
view. Right, dorsomedial view. A, Xenodon mnrremi; B, Heterodon simus; C,
Ieterodon nasicus; D, Heterodon platyrhinos; E, Heterodon platyrhinos; F, Heter-
odon nasicus; G, Heterodon simus; H, Xenodon merremni.
PALATINES. The palatine lies anterior to the pterygoid. A large an-
teriorly-arched medial process near the middle of the bone is curved
ventroposteriorly at its tip. The medial palatine process extends
anteriorly nearly to the vertical posterior process of the vomer. In
Heterodon it makes contact with the spenoid, as in Xenodon or more
generalized colubrids such as Elaphe or Thamnophis. The medial
arched process curves ventrally, but not posteriorly in Xenodon. A
smaller lateral process expands posteriorly on the anterior end.
ECTOPTERYGODS. The ectopterygoids (fig. 5) are dorsoventrally flat-
tened and forked anteriorly. The forked end lies over the posterior
end of the maxillary. They are longer relative to the basioccipital
in H. platyrhinos than in H. simus or H. nasicus. The forked anterior
ends are bent sharply media in Xenodon as opposed to the simple
"Y" in Heterodon.
BULLETIN FLORIDA STATE MUSEUM
THE MANDIBULAR UNIT
This unit consists of two mandibles, two quadrates, and two supra-
SUPRATEMPORALS. The supratemporals are laterally flattened and
approximately half the length of the quadrates. The posterior ends
are medially twisted to allow for the medial angle of the quadrate.
The anterior ends of the supratemporals lie against the parietal pos-
terior to the vertical lateral process. The posterior ends extend over
the pro-otic and are supported by the dorsally inclined lateral pro-otic
ridge which restricts movement in a ventral arc. In Xenodon the
supratemporals are longer than half the quadrate length and are not
QUADRATE. The quadrates (fig. 7) are elongate, flattened proximally
for the supratemporal articulation, and connected to the mandibles
by a saddle joint. The proximal edges are expanded posteriorly into
a rounded flange. A weak vertical process occurs on the posterior
edge of each shank, about one third the length of the bone from the
distal end, for a cartilaginous connection with the columella. In
Figure 7. Quadrate bone of Heterodon and Xenodon. A, Heterodon platy-
rhinos; B, H. nasicus; C, H. simus; D, Xenodon merremi. Proximal end of bone
1965 CRANIAL ANATOMY OF THE HOG-NOSED SNAKES 287
Xenodon the proximal end is truncated and the vertical process on
the shank is strongly developed.
MANDIBLE. The mandibles (fig. 8) consist of four bones, the com-
pound, dentary, angular, and splenial. Only the dentary bears teeth.
The angular is fused to the anterior ventrolateral border of the com-
pound bone. The sliver-like splenial lies on the medial posterior sur-
face of the dentary and is pierced at the posterior end by a foramen.
Figure 8. The mandible of Heterodon and Xenodon. A, Heterodon platy-
rhinos (medial view); B, Heterodon platyrhinos (lateral view); C, Xenodon mer-
remi (medial view); D, Xenodon merremi (lateral view). Abbreviations: as, ar-
ticular surface for quadrate; an, angular; c, compound; d, dentary; il, inner
lamella; ol, outer lamella, Mandibular fossa located between outer and inner
BULLETIN FLORIDA STATE MUSEUM
The dentary is relatively shorter with respect to the compound bone
in Heterodon than in Xenodon, Elaphe, Coluber, or Thamnophis.
As the laminae are weakly developed, the mandibular fossa is shal-
low. In H. nasicus, and to some degree in H. simus, the inner lamina
is conspicuously higher and more pointed than in H. platyrhinos.
However (Auffenberg, 1955) has shown the shape of the laminae to
vary ontogenetically in Coluber. The medial side of the inner sur-
angular lamina is recessed for attachment of the adductor posterior
profundus and the lateral head of the pterygoideus. In Heterodon
the mandible is relatively shorter than in Xenodon and the angular
bone is very thick and offset medially along its entire length. The
dentary of Xenodon is approximately half the mandibular length, as
opposed to one third the mandibular length in Heterodon. The lat-
eral dentary foramen extends anteriorly as a shallow groove in Xeno-
don and the mandibular fossa is shallow, as in Heterodon. Consider-
able lateral movement of the dentary is possible in both genera.
THE HYom UNIT
The hyoid apparatus is shaped like a hairpin with the loop ante-
rior. In Heterodon the basihyal is round and has no anterior projec-
tion. In Xenodon a cartilaginous anterior projection from the basi-
hyal is present, as it is in Vipera palestinae and Bothrops. I could
find no separation into a first or second ceratohyal as Cowan and Hick
(1951) reported in Thamnophis. The pattern of the hyoid apparatus
fits Smith's (1948) classification which combines the Colubridae, Sol-
enoglypha, Elapidae, and Hydrophiidae into a single group with re-
gard to hyoid configuration.
THE VERTEBRAL UNIT
This unit is not strictly within the scope of this paper; it is treated
only where the description of muscle attachments warrants.
THE ADDUCTORES MANDIBULAE
The fibers of these muscles are directed dorsoventrally except
those of the pterygoideus which are more or less horizontal to the
longitudinal plane of the skull. Embryologically the adductors are
derived from the adductor externus medialis. This suggests that the
sauropsidians evolved from forms having a single adductor which
subsequently split into three major parts. In turn each of the major
1965 CRANIAL ANATOMY OF THE HOG-NOSED SNAKES 289
parts may undergo various divisions (Edgeworth, 1935). The ad-
ductores mandibulae unite the mandibular unit of the skull with the
cranial and palato-maxillary unit.
M. ADDUCTOR EXTERNUS SUPERFICrALIS. This is the most lateral cran-
ial muscle. It originates from the posterior side of the postfrontal
and passes medially to the large Harderian gland and around the
posterior angle of the mouth to the dorsolateral posterior end of the
dentary. This muscle differs from its counterparts in Thamnophis,
Elaphe, and Natrix in that: (1) it curves anteriorly after passing the
posterior mouth angle and (2) it is not associated with an aponeuro-
sis. The forward curve of this muscle (fig. 9) is reminiscent of the
vipers and was used by Smith (1964) as one of the characters of the
Heterodontinae. In Xenodon the muscle lies over the dorsolateral
surface of the Harderian gland. It extends posteroventrally from the
postfrontal to the anterior lateral surface of the adductor externus pro-
fundus where it becomes involved in a large aponeurosis. Haas
(1931) reported some fibers curving forward around the posterior
Figure 9. Lateral view of superficial cranial muscles of Heterodon and
Xenodon. A, Heterodon platyrhinos; B, Xenodon rabdocephalus; Abbreviations:
aes, adductor externus superficialis; aep, adductor externus profundus; aem, ad-
ductor externus medialis; p, pterygoideus; r, retractorquadrati; cvq, cervico-qua-
dratus; h, harderian gland; o, orbit; o-q-m, depressor mandibulae.
BULLETIN FLORIDA STATE MUSEUM
angle of the mouth to insert on the mandible in X. merremi, a condi-
tion I found duplicated in X. severus. Specimens of X. rabdocephalus
show no clear-cut mandibular attachment.
M. ADDUCTOR EXTERNUS MEDIALIS. This muscle originates from the
posterolateral surface of the parietal. Its fibers are closely associated
with the medial posterior border of the adductor externus superfi-
cialis and the medial anterior border of the adductor externus pro-
fundus. The insertion is on the compound bone posterior and me-
dial to the insertion of the adductor externus superficialis. This mus-
cle is more distinct in H. simus and H. nasicus than in H. platyrhinos.
In Xenodon the adductor externus medialis is very distinct and makes
a slight bend forward as it does in Heterodon.
M. ADDUCTOR EXTERNUS PROFUNDUS. This is a thick triangular muscle
with the apex at the quadrato-mandibular joint. It arises from the
anterior lateral surface of the quadrate and inserts on the lateral bor-
der of the mandibular fossa forward and lateral of the insertion of
the adductor externus superficialis.
M. ADDUCTOR POSTERIOR SUPERFICIALIS. This thin muscle arises from
the entire anterior surface of the quadrate and inserts in the mandibu-
lar fossa. The anterior border is bounded by the mandibular branch
of the trigeminal nerve.
M. ADDUCTOR POSTERIOR PROFUNDUS. This thick columnar muscle orig-
inates from the medial surface of the quadrate and inserts on the pos-
terior medial border of the mandibular fossa. In Xenodon it is tri-
angular, as in Elaphe and Thamnophis.
M. PTERYGOIDEUS. This heavy, two-headed muscle arises from the
lateral arm of the ectopterygoid via a strap-like tendon. The lateral
head extends posteroventrally from the dorsal surface of the tendon
to the anterior medial side of the mandibular fossa. The mandibular
branch of the fifth cranial nerve separates the posterior edge of this
slip from the anterior border of the adductor posterior superficialis.
The posterior head is the largest part of the muscle and forms a con-
spicuous belly near its origin on the posterior medial end of the com-
pound bone and the quadrate-compound joint. In Xenodon the lat-
eral head is reduced.
M. PTERYGOIDEUS ACCESSORIUS. The pars posterior is a flat muscle
originating from the pterygo-ectopterygoid joint and the adjacent
portion of the pterygoid. It extends posterolaterally to insert on the
medial posterior end of the compound bone.
1965 CRANIAL ANATOMY OF THE HOG-NOSED SNAKES 291
M. PSEUDOTEMPORAIS. This thin flat muscle lies medial to the ptery-
goideus accessorius. The origin is the posterior dorsal edge of the
postfrontal. It extends posteroventrally to the dorsolateral surface of
the compound bone anterior to the lateral head of the pterygoideus.
This muscle is best developed in H. nasicus and H. simus.
THE CONSTRICTOR DORSALIS
The constrictor dorsalis (fig. 10) is derived embryologically from
the dorsal masticatory muscle plate which subsequently divides into
slips inserting on the pterygoid, vomer, and quadrate. The simplest
condition of this group is found in Sphenodon, a rhyncocephalian
(Edgeworth, 1935). The presence of a constrictor dorsalis is charac-
teristic of forms with kinetic skulls, and its degree of development
in reptiles reflects the degree of cranial kinesis (Ostrom, 1962). The
constrictor dorsalis unites the cranium with the mandibular unit, the
palatomaxillary unit, and the nasal unit.
Figure 10. Ventral view of cranial musculature in Heterodon and Xenodon.
A, Heterodon platyrhinos; B, Xenodon rabdocephalus; Abbreviations: ds, dorsal
slip of protractor pterygoidei (pp removed on right side); pa, pterygoideus
acessorius; p, pterygoideus; pp, protractor pterygoidei; pq, protractor quadrati;
rp, retractor pterygoidei; rv, retractor vomeris.
BULLETIN FLORIDA STATE MUSEUM
M. PROTRACTOR QUADRATI. The protractor quadratus is a flat, thin
muscle arising from the fascia over the ventral semispinalis muscles.
It extends posterolaterally to the distal end of the quadrate and lies
dorsal to the entire length of the protractor pterygoideus. In Xeno-
don it arises posterior to the protractor pterygoidei and extends lat-
erally, dorsally overlapping the latter only with its distal end.
M. PROTRACTOR PTERYGOIDEI. This strap-like muscle issues from the
midline of the sphenoid anterior to the sphenoid-basioccipital junc-
tion. It extends posterolaterally to the medial side of the pterygoid
near the pterygoid-quadrate junction. In Xenodon the muscle at-
taches medially to the pterygoid from the end of the tooth row to
the distal end of the bone. In addition all the Xenodon examined
had a thin, narrow muscle located dorsally to the protractor ptery-
goidei. This muscle, here considered a slip of the protractor ptery-
goidei, originates from the lateral sphenoid border anterior to the
pro-otic and extends posterolaterally to the dorsomedial surface of the
pterygoid at a point medial and midway to the insertion of the leva-
M. LEVATOR PTERYGODEI. This fan-shaped muscle arises from the lat-
eral vertical wall of the parietal medial to the dorsal end of the post-
frontal. The ventrally directed fibers fan out posteriorly for inser-
tion on the posterior dorsolateral edge of the pterygoid. The wide
insertion extends from the rear pterygoid teeth to the distal end of
M. RETRACTOR PTERYGOIDEI. The origin of this thick muscle is the
groove extending ventroanteriorly through the pro-otic and sphenoid
bones. It extends anteroventrally to the medial border of the ptery-
goid and palatine bones at their junction. The muscles arises from
the sphenoid midline in Xenodon, Thamnophis, Elaphe, and Natrix
(Cowan and Hick, 1951; Adams, 1925; Albright and Nelson, 1959).
A high origin, not unlike the one in Heterodon, exists in Crotalus
adamanteus and Agkistrodon picivorus among the vipers.
M. RECTRACTOR VOMERIS. This short cylindrical muscle originates from
the sphenoid anterior to the origin of the protractor pterygoidei. The
insertion is on the ventral posterior end of the vomer.
THE INTERMANDIBULAR MUSCLES
The intermandibular muscles are presumed to have developed
from a primordial single muscle sheet as indicated by the common
midline insertion of the intermandibularis anterior and posterior (Cow-
1965 CRANIAL ANATOMY OF THE HOG-NOSED SNAKES 293
an and Hick, 1951; Edgeworth, 1935). The intermandibular muscles
are associated only with the mandibular unit.
M. INTERMANDIBULARIS ANTERIOR. This is a two-headed muscle arising
from a bar of connective tissue that lies on the midline between the
anterior tips of the mandibles. The dorsal head extends vertically
to the lateral anterior tip of the tongue sheath. The lateral head is
directed anterolaterally to the ventromedial tip of the dentary. No
glandular portion was detected as reported for Elaphe (Albright and
Nelson, 1959) or Thamnophis (Cowan and Hick, 1951). However a
short muscle that originates from the connective tissue bar and ex-
tends posteriorly to enclose the posterior end of the sublingual gland
may represent the glandular portion these workers describe.
M. TRANSVERSUS BRACHIALIS. This narrow, thin muscle arises from
the skin between the origin of the intermandibularis posterior, pars
posterior, and intermandibularis anterior. It extends dorsally in an
anterolateral direction to insert by two heads at (1) the ventral poste-
rior end of the sublingual gland and (2) the mucus membrane cover-
ing the floor of the mouth. The glandular attachment is the stronger.
M. INTERMANDIBULARIS POSTERIOR: PARS ANTERIOR. This is a flat
muscle which originates from the midline at a level approximately
even with the dentary-angular joint. A few fibers arise from the
posterior end of the sublingual gland and merge in their anterior
length with the glandular attachment of the transversus brachialis.
The muscle extends posterolaterally, dorsal to the neuro-costo-man-
dibularis, to the ventral edge of the compound bone opposite the
anterior border of the mandibular fossa.
M. INTERMANDIBULARIS POSTERIOR: PARS POSTERIOR. This straplike
muscle lies ventral to the neuro-costo-mandibularis. It arises from
the midline and extends posterolaterally to the posterior lateral side
of the compound bone at a level even with the posterior border of
the mandibular fossa.
THE HYPOBRANCHIAL SPINAL MUSCLES
The hypobranchial musculature develops from downgrowths of
two or more spinal myotomes. The resulting primordium divides
into an anterior geniohyoideus and a posterior rectus-cervicis. The
tongue is formed from a median elevation of the floor of the mouth.
The hyoglossus is derived from the hypobranchial primordium before
separation. The anterior geniohyoideus becomes the geniotrachealis
and the genioglossus. The hyolaryngeus arises from the posterior
BULLETIN FLORIDA STATE MUSEUM
end of the hyoglossus in Tropidonotus-Natrix, (Edgeworth, 1985).
These muscles are essentially the extrinsic tongue muscles; some of
their members unite the hyoid and mandibular skull units.
M. HYOGLOSSUS. The tongue consists of two adjacent muscles which
are bifurcated anteriorly. They arise from the posterior ends of the
ceratohyals and extend anteriorly to become sheathed in the extrinsic
M. GENIOGLOSSUS. This is a cylindrical two-headed muscle originat-
ing from (1) the anterior tip of the midline cartilage and (2) the ante-
rior medial end of the dentary in conjunction with the geniotrachealis.
The anterior head (1) extends posteriorly along the lateral side of
the tongue. The anterior medial head (2) passes posteromedially
from the dentary and merges with the anterior head at a point even
with the dentary-angular joint. The lateral head constitutes the dorso-
lateral half of the tongue sheath and the anterior head forms the ven-
M. GENIOTmACHEALIS. This muscle originates in conjunction with the
lateral head of the genioglossus from the anterior tip of the dentary.
It passes posterodorsally to the dorsolateral area of the trachea at a
point posterior to a line drawn through the dentary-compound joints
of the mandibles.
M. HYOLARYNGEUS. This thin strand of muscle arises from fascia over
the Neuro-costo-mandibularis between the ceratohyals and the pos-
terior end of the mandible. It extends anterodorsally to the trachea
where its fibers become closely applied to the dorsal surface of the
insertion of the geniotrachealis.
M. CERVICO-HYOIDEUS. This broad sheet of fibers arises from the
gastro-steges in the anterior trunk region and inserts on the basihyal
and anterior ceratohyals.
M. CERVICO-QUADRATUS. This thin, flat muscle arises from skin and
fascia over the costal head of the neuro-costo-mandibularis. It ex-
tends anterodorsally, medial to the retractor quadrati, to attach to
the posterior edge of the quadrate. This muscle is better developed
in H. nasicus than in the other species of Heterodon.
M. CUTANEO-QUADRATUS. This is a thin flat muscle arising from the
skin around the distal end of the quadrate, lying lateral to the re-
tractor quadrati, and inserting on the mid-proximal end of the
quadrate. This muscle is best developed in H. nasicus. Haas (1931)
1965 CRANIAL ANATOMY OF THE HOG-NOSED SNAKES 295
reported it in Xenodon merremi, and I have found it in X. severus
and X. rabdocephalus. I have not found it in Elaphe or Thamnophis.
THE HYom MuscLES
The hyoid muscle plate lies in front of the trunk myotomes as a
vertical band of cells extending posteriorly and giving rise to the con-
strictor colli. The anterior fibers of the colli separate to form sev-
eral muscles. In the Ophidia the depressor mandibulae, the retractor
quadrati, and the neuro-costo-mandibularis are the products of this
separation (Edgeworth, 1935; Romer, 1956). The hyoid musculature
unites the mandibular and vertebral skull units.
M. REtRACTOR QUADRATI. This is a wide flat muscle arising from fascia
over the spinalis-semispinalis muscles at the level of vertebrae VI
to IX in H. simus and H. platyrhinos, and from vertebrae IV to VI in
H. nasicus. In all species of Heterodon the insertion is on the distal,
lateral side of the quadrate and over the quadrate-compound joint.
Both origin and insertion appeared constant in Heterodon. Albright
and Nelson reported variations at the insertion in Elaphe. In Xeno-
don merremi it is two headed at its insertion, one head attaching to
the inner band of the quadrate-maxillary ligament, the other making
a normal insertion (Haas, 1931; Anthony and Serra, 1951). The ar-
rangement of these muscle heads is similar in both Xenodon severus
and X. merremi. In X. rabdocephalus the muscle does not show a
clear-cut division into two heads, but its anterior part attaches to the
inner band of the ligament at its insertion.
M. DEPRESSOR MANDIBULAE. This thick muscle lies on the posterior
edge of the quadrate. The origin is from the supraoccipital by a few
strands of connective tissue, and from the posterior border of the
quadrate. The insertion is the quadrate-mandibular joint.
M. NEURO-COSTO-MANDIBULARIS. This large, flat muscle arises from
three heads. The vertebral head is from the dorsal surface of the
trunk posterior to the origin of the retractor quadrati, the costal head
from the distal end of the first few ribs, and the hyoid head from the
lateral border of the anterior ceratohyals. All heads merge ventrally
at a point anterior to a line through the quadrate-mandibular joints.
The insertion is on the medioventral border of the mandible.
M. CONSTRCTroR COLLI. This thin, subcutaneous muscle is closely ap-
plied to the skin in the neck region. It encircles the anterior trunk
region and extends rostrad on the ventral surface as far as the mandib-
ular attachment of the intermandibularis posterior pars posterior.
BULLETIN FLORIDA STATE MUSEUM
THE GLANDS AND LIGAMENTS
No attempt is made to describe these structures in detail. Those
that seem especially pertinent are described briefly.
LABIAL GLANDS. The labial glands lie under the skin of the sides
of the mouth. The inferior gland is moderately enlarged in Hetero-
don. In H. simus and H. nasicus the superior gland is markedly en-
larged posteriorly and the parotid element is well differentiated. In
Xenodon severus and X. rabdocephalus the posterior superior gland
is not usually enlarged, but the parotid gland is well developed and
a pale cream color in preserved specimens.
QUADRATE-MAXILLARY LIGAMENT. This is a flat thin band divided
into two parts anteriorly. The ligament arises from quadrate-mandib-
ular capsule and passes rostrally to attach by a medial band to the
posterior end of the upper labial gland and by a lateral band to the
skin over the posterior end of the upper labial gland. In Xenodon
the ligament is also doubled, but the inner band connects to the pos-
terior end of the maxillary bone and the lateral band attaches to the
rear of the parotid gland. The maxillary attachment is present in
Natrix, Thamnophis, and Elaphe. In Xenodon the anterior part of
the retractor quadrati attaches to the inner band near its origin.
POSTFRONTAL-MAXILLARY LIGAMENT. This tendon extends from the
distal end of the postfrontal to the ventrolateral side of the maxillary
anterior to the maxillary-ectopterygoid joint.
PALATO-MAXILLARY LIGAMENT. This structure provides a tendonous
connection between the anterior lateral process of the palatine and
the anterior medial process of the maxillary.
HARDERIAN GLAND. This is a large, oval gland projecting from the
posterior ventral floor of the orbit medial to the postfrontal (fig. 9).
The upturned, projecting premaxillary and the compact nasal
structure reflect Heterodon's burrowing behavior (Bogert, 1947). In
his study of Heterodon's burrowing Davis (1946) described the modi-
fied rostrum as a double-shared plow that forces soil to the side. This
structure is not only used in burrowing but also to procure food.
Goin (1947) reports that Heterodon simus digs up spadefoot toads
(Scaphiopus h. holbrooki) with its snout. I have watched H. platy-
rhinos seek and root out common toads (Bufo terrestris). The elon-
1965 CRANIAL ANATOMY OF THE HOG-NOSED SNAKES 297
gate nasals and the series of widened dorsal cranial bones provide
a median line of support, while the shortened head has an increased
mechanical advantage and is thus more efficient for rooting.
The enlarged ungrooved caudal maxillary teeth are separated by
a diastema from the smaller anterior teeth in both Heterodon and
Xenodon. These enlarged teeth are directed posteriorly and medially
and rise from the maxillary at such an angle that the end of the tooth
is nearly parallel to the bone. This suggests the existence of a mech-
anism for moving the tooth tips to a more perpendicular position.
Boulenger (1915) describes a forward rotation of the maxillary in
X. merremi to engage the enlarged teeth in solenoglyph fashion.
Haas (1931) suggests that the postfrontal-maxillary ligament prevents
the vertical rotation Boulenger described and ascribes the erection
of the enlarged teeth to a lateral rotation of the maxillary (see also
Anthony and Serra, 1951). Albright and Nelson (1959) describe a
lateral rotation of the maxillary when Elaphe opens its mouth. I
have noticed that a similar movement in Heterodon brings the en-
larged maxillary teeth into play. The ventrolateral attachment of
the postfrontal-maxillary ligament increases the lateral rotation of
the maxillary. A slight lateral movement of the postfrontals increases
the lateral rotation of the maxillaries and widens the lateral gape be-
tween their posterior ends. This increase in gape helps the snake
puncture inflated toads.
Enlargement of the posterior position of the upper labial gland is
often accompanied by modifications of the maxillary teeth (Smith
and Bellairs, 1947). In view of the well developed parotid gland in
Heterodon, Bragg's report (1960) of toxic effects from a H. nasicus
bite is not surprising. W. T. Neill (personal communication) reported
a fence swift, Sceloporus undulatus, killed by the apparently toxic
bite of H. simus. The snake seized the lizard by the thing leg, chewed,
and imbeded its rear maxillary teeth. It held the prey for less than
a minute. The lizard was dead when the snake released it to seize
McAlister (1953) investigated the toxic effects of the parotid gland
of H. platyrhinos and found that after injection of a parotid solution
anurans died within 24 hours but mice were unaffected. The pro-
nounced enlargement of the superior labial gland in H. simus and
H. nasicus compared to those of other snakes and its relative nor-
mality in H. platyrhinos offer a possible explanation of the contrary
findings. The morphology of the gland and the reports published
so far suggest that all Heterodon species are mildly toxic. In H.
simus and H. nasicus the toxin is probably useful in subduing prey.
BULLETIN FLORIDA STATE MUSEUM
There is (Stejneger, 1895) at least one record of a poisonous bite
from Xenodon severus, although a bite by X. rabdocephalus produced
no ill effects.
The cranial osteology of Heterodon supports the relationship of
H. simus and H. nasicus and confirms a similar conclusion based on
vertebral characters (Auffenberg, 1963). Table 1 lists the major points
of comparison and indicates the groupings. The greater number of
osteological differences may be attributed to the fact that muscles
are more protean than bones and consequently less easily compared.
Kochva (1962), however, found significant intrageneric variations in
musculature in Vipera, as did Haas in Causus (Haas, 1952).
TABLE 1. MYOLOGICAL AND OSTEOLOGICAL COMPARISONS WITHIN THE
GENUS HETERODON *
H. platyrhinos H. simus H. nasicus
Premaxillary X X
Maxillary X X
Nasal X X
Ectopterygoid X X
Pterygoid X X
Frontals X X
Parietals X X
Supraoccipital X X
Orbital foramen X X
Foramen magnum X X
Adductor extenus medialis X X
Pseudotemporalis X X
Retractor quadrati X X
Cutaneo quadratus X X
Retractor quadrati X X
Labial glands X X
The X's indicate the species that
in the column at the left.
are most similar for the structures listed
SThe increased osteological specializations in H. simus and in H.
nasicus indicate that these species represent advanced Heterodon
evolution. Thus the more generalized species, H. platyrhinos, is prob-
ably representative of the generic ancestor. The presence of a bi-
lobed orbital foramen in H. platyrhinos and Xenodon and its gradu-
ated diminuation in H. simus and H. nasicus respectively add sup-
port to this conclusion.
1965 CRANIAL ANATOMY OF THE HOG-NOSED SNAKES 299
Other than the gross features mentioned by Smith uniting the
Heterodontinae and Dunn's characters for the Ophiinae, Heterodon
and Xenodon have cranial characters in common. The prefrontals and
to some degree the postfrontals, the maxillary and the method of en-
gaging the maxillary teeth, the sphenoid, the shape of the orbital for-
amen, and the trunk musculature (see below) leave little doubt about
the Heterodon-Xenodon affinity. This does not imply that other
genera are excluded from this grouping. The presence of an apo-
neurosis on the lateral jaw musculature, the more typically colu-
brid features of the head, and the colubrid tendency of the trunk
musculature point to the general colubridization of Xenodon as
opposed to Heterodon.
When considering the relationships of Heterodon to other genera
one must distinguish specialized features from those having phylo-
genetic or taxonomic importance. It is also necessary to estimate
what effect, if any, osteological changes have on associated muscula-
ture. In Heterodon the major areas of specialization are the nasal
bones and the median series of dorsal cranial bones. The anterior
lateral and ventral cranial elements remain rather unspecialized. The
pre- and postfrontals, the maxillaries, the sphenoid, and the orbital
foramen show characters of some phylogentic or taxonomic impor-
The maxillaries are an important diagnostic tool because they
are usually the site of dental modifications and ordinarily carry any
specialized teeth that may occur (Bogert, 1948). The pre- and post-
frontals and the sphenoid are similar in both genera, as is the orbital
foramen. I believe this similarity has at least some taxonomic import.
The skull specializations of Heterodon are illustrated in the widen-
ing of the frontals, parietals, and supraoccipitals, and in the extreme
shortening of the skull behind the postfrontals. This shortening is
most obvious in the incorporation of the various small pro-otic fora-
mina into the anterior pro-otic foramen, and in the loss of the dorso-
lateral parietal groove, which is present in Xenodon and other unspe-
cialized colubrids such as Thamnophis, Elaphe, and Coluber. The
loss of this groove might be expected to alter the adductor externus
muscles which normally arise from it, but the effect is slight. Only
the adductor externus medialis suffers, being smaller in Heterodon
than in Xenodon and the above genera, which places the adductor
externus superficialis and profundus closer together. The presence
of a large and well developed adductor externus superficialis in Het-
erodon is not correlated with osteological specializations of the skull.
BULLETIN FLORIDA STATE MUSEUM
The development of this muscle may actually be correlated with the
absence of a well developed aponeurosis.
The shortened maxillary and its enlarged caudal teeth, the highly
developed parotid gland, and the viper-like appearance of Hetero-
don invite speculation on the role of the xenodontine snakes in the
evolution of the solenoglyphs. Anthony (1955) notes two points for
fang location, a maxillo-prefrontal locus (proterodont) and a maxillo-
transverse locus (opisthodont). The vipers developed along the
opisthodont line from aglyphous colubrids. Heterodon and Xenodon
demonstrate the opisthodont condition. Haas (1952) arrived at a
similar conclusion on studies centered around Causus; he hypothe-
sized that the ancestral solenoglyph would be a primitive colubrid
with a double adductor externus superficialis, whose posterior mem-
ber being derived from the adductor externus profundus gave rise
to the gland muscle. He cited Sibon as an example close to the hypo-
thetical ancestor. In a later paper (1962) Haas considered the elevator
anguli oris (arising from the postfrontal and inserting at the posterior
corner of the mouth) a primitive character of the lateral jaw muscula-
ture, and one probably present in the viper ancestor. Heterodon and
Xenodon show an advanced colubrid plan of lateral jaw musculature
since the elevator anguli oris is missing. However both show a prim-
itive characteristic in the forward curving of the adductor externus
More impressive is Mosauer's observation of the similarity between
the trunk musculature of Heterodon platyrhinos and Crotalus horri-
dus. My dissections, which confirm those of Mosauer, also show that
the trunk musculature of Xenodon (rabdocephalus) resembles that of
Crotalus. Both Xenodon and .Heterodon are similar to Crotalus
which is similar to Vipera, Causus, and Agkistrodon) in the fibrous
association of the semispinalis and the interarticularis superior (per-
sonal observation, not in Mosauer), the osseous attachment of the
semispinalis to the neural arch (a tendonous attachment to the longis-
simus in Coluber and the other advanced colubrids), and in a fibrous
association of the spinalis and the multifidus. The trunk musculature
of Heterodon is very close to the pre-Colubrid stage "F" of Auffenberg
(1961). Xenodon differs from Heterodon pnly in the increased length
of the muscle fibers, particularly in the spinalis complex. This length-
ening of fibers is a step in the colubridization of trunk musculature
Haas (1952) stresses the "logical necessity" of double adductor ex-
ternus superficialis in the viper ancestors and points out the fruitless-
ness of relying on osteological characters alone. I believe osteology
1965 CRANIAL ANATOMY OF THE HOG-NOSED SNAKES 301
must be considered because profound osteological changes eliminate
sites for muscular attachment and force muscles to conform to a new
topography. The shortened posterior of the skull in Heterodon places
the adductor externus superficialis and profundus close together. From
this condition the development of a gland muscle from an anterior
splitting off of the profundus seems reasonable. A subsequent an-
terior migration of the insertion of the separated member, largely
accomplished in the existing single adductor externus profundus of
Heterodon, and a posterior migration of the superficialis would form
the characteristic muscular loop seen in the solenoglyphs. This path
of gland-muscle development eliminates the necessity of a double
adductor externus superficialis in the viper ancestor.
While head musculature must be considered in solenoglyph phy-
logeny, the trunk myology appears more reliable. The extreme short-
ening of the maxillary and the corresponding elongation of the ecto-
pterygoid, the development of a poison gland and of a compressor for
the gland impose severe changes in the muscular construction of the
head. In a sense these changes are adaptive and tend to mask phylo-
genetic affinities. In the absence of such drastic osteological changes
in the vertebral structure, the trunk myology is more conservative
and reflects evolution more accurately. The occurrence of an aglyph-
ous colubrid with enlarged maxillary teeth in the opisthodont posi-
tion, with at least one primitive characteristic of the lateral jaw mus-
culature and a viperid-like trunk myology, merits serious consider-
ation as a viper ancestor. In view of these characteristics, it is con-
ceivable that a xenodontine protoviper gave rise to (1) Xenodon (ad-
vanced Heterodontinae or Xenodontinae), (2) a Heterodon type which,
although it became specialized, retained many primitive features, and
(3) the solenoglyphs.
It is hypothesized that the xenodontine proto-viper possessed the
following characters: (1) a viperid trunk musculature with short fibers
in the spinalis complex, (2) enlarged opisthodont maxillary teeth and
a corresponding shortening of the maxillary, (3) a well developed par-
otid gland, (4) a well developed adductor externus superficialis closely
associated with the adductor externus profundus, (5) a rostrally ex-
tended insertion of the adductor externus ppofundus, (6) the presence
of a elevator anguli oris, and (7) no aponeurosis involving the adductor
externus superficialis, thus allowing that muscle to make a fleshy in-
sertion on the mandible.
Heterodon resembles this condition except that it lacks a elevator
anguli oris. Xenodon is farther along the colubridization process,
retaining only the enlarged opisthodont maxillary teeth on a short-
BULLETIN FLORIDA STATE MUSEUM
ened maxillary (point 2 above), and a well developed parotid gland
(point 3 above) and to some extent the close association of the ad-
ductor externus superficialis and profundus (point 4 above).
Adams, L. A.
1925. Correlation of the musculature and the movement of the Skull in Natrix,
with some suggestions of homology in the Lacertilians. Jour. Morph.,
Albright, R. G., and E. M. Nelson
1959. Cranial kinetics of the generalized colubrid snake Elaphe obsoleta
quadrivittata. I. Descriptive morphology. Jour. Morph., 105: 193-240.
1959. Cranial kinetics of the generalized colubrid snake Elaphe obsoleta
quadrivittata. II. Functional morphology, 105 (2): 241-292.
1955. Essai sur l'Evolution anatomique de l'appareil venimeux des Ophidiens.
Annales des Sciences Naturelles, Zoologie, 17 (1): 7-53.
Anthony, J., and R. G. Serra
1951. Anatomie de l'appareil de la morsure chez Xenodon merremi B., ser-
pent aglyphe de l'Amerique tropical. Arquivos do Museum Nacional,
Rio de Janeiro, 42 (1): 21-47.
1955. The status of the snake Coluber acuminatus. Copeia (1): 65-67.
1961. Additional remarks on the evolution of the trunk musculature in snakes.
Amer. Midland Nat. 65 (1): 1-16.
1962. A review of the trunk musculature in the limbless land vertebrates. Am.
Zoologist, 2: 183-260.
1963. The fossil snakes of Florida. Tulane Studies in Zool., 10 (3): 131-216.
Bogert, C. M.
1940. Herpetological results of the Vernay Angola Expedition. Bull. Am.
Mus. Nat. Hist., 77: 1-107.
1943. Dentitional phenomena in cobras with notes on adaptive modifications
of fangs. Bull. Am. Mus. Nat. Hist., 81: 285-360.
1947. The status of the genus Leptodrymus Amaral, with comments on modi-
fications of colubrid premaxillae. Am. Mus. Nov., 1352: 1-14.
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3 1262 04394876 8
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