SKELETON
The typical Ophidian skull is characterized by a solidly ossified
brain-case, with the distinct frontals and the united parietals
extending downwards to the basisphenoid, which is large and produced
forward into a rostrum extending to the ethmoidal region. The nasal
region is less completely ossified, and the paired nasals are often
attached only at their base. The occipital condyle is either trilobate
and formed by the basioccipital and the exoccipitals, or a simple knob
formed by the basioccipital; the supraoccipital is excluded from the
foramen magnum. The basioccipital may bear a strong, curved ventral
process or hypapophysis (in the Vipers).
The prefrontal is situated, on each side, between the frontal and the
maxillary, and may or may not be in contact with the nasal; the
postfrontal, usually present, borders the orbit behind, rarely also
above, and in the Pythons a supraorbital is intercalated between it and
the prefrontal.
[Illustration: FIG. 3--SKULL OF _Python amethystinus_. (From British
Museum Catalogue of Snakes)
_an_, Angular; _ar_, articular; _bo_, basioccipital; _bs_,
basisphenoid; _cor_, coronoid; _c.a_, columella auris (stapes); _d_,
dentary; _eo_, exoccipital; _epg_, ectopterygoid (transverse); _f_,
frontal; _m_, maxillary; _n_, nasal; _p_, parietal; _pl_, palatine;
_pm_, premaxillary; _prf_, prefrontal; _pro_, proötic; _pg_,
pterygoid; _ptf_, postfrontal; _q_, quadrate; _so_, supraoccipital;
_sor_, supraorbital; _sp_, splenial; _ste_, supratemporal; _tu_,
turbinal; _v_, vomer.]
The premaxillary is single and small, and as a rule connected with the
maxillary only by ligament. The paired vomer is narrow. The palatine and
pterygoid are elongate and parallel to the axis of the skull, the latter
diverging behind and extending to the quadrate or to the articular
extremity of the mandible; the pterygoid is connected with the maxillary
by the ectopterygoid or transverse bone, which may be very elongate, and
the maxillary often emits a process towards the palatine, the latter
bone being usually produced inwards and upwards towards the anterior
extremity of the basisphenoid. The quadrate is usually large and
elongate, and attached to the cranium through the supratemporal (often
regarded as the squamosal). In rare cases (_Miodon_, _Polemon_) the
transverse bone is forked, and articulates with two branches of the
maxilla. The quadrate and the maxillary and palatopterygoid arches are
more or less movable to allow for the distension required by the passage
of prey, often much exceeding the calibre of the mouth. For the same
reason, the rami of the lower jaw, which consist of dentary, splenial,
angular, and articular elements, with the addition of a coronoid in the
Boidæ and a few other small families, are connected at the symphysis by
a very extensible elastic ligament.
The hyoid apparatus is reduced to a pair of cartilaginous filaments
situated below the trachea, and united in front.
There are various modifications according to the genera. A large vacuity
may be present between the frontal bones and the basisphenoid
(_Psammophis_, _Coelopeltis_); the maxillary may be much abbreviated and
movable vertically, as in the Viperidæ; the pterygoids may taper and
converge posteriorly, without any connexion with the quadrate, as in the
Amblycephalidæ; the supratemporal may be much reduced, and wedged in
between the adjacent bones of the cranium; the quadrate may be short or
extremely large; the prefrontals may join in a median suture in front of
the frontals; the dentary may be freely movable, and detached from the
articular posteriorly.
[Illustration: FIG. 4--SKULL OF _Typhlops lumbricalis_. (From British
Museum Catalogue of Snakes)
Lettering of the bones as in Fig. 3]
[Illustration: FIG. 5--SKULL OF _Glauconia macrolepis_. (From British
Museum Catalogue of Snakes)
Lettering of the bones as in Fig. 3]
The deviation from the normal type is much greater still when we
consider the degraded, worm-like members of the families Typhlopidæ
(Fig. 4, p. 43) and Glauconiidæ (Fig. 5), in which the skull is very
compact and the maxillary much reduced. In the former this bone is
loosely attached to the lower aspect of the cranium; in the latter it
borders the mouth, and is suturally joined to the premaxillary and the
prefrontal. In both the tranverse bone and the supratemporal are absent,
but the coronoid element is present in the mandible.
[Illustration: FIG. 6--SKULL OF _Tropidonotus natrix_. (From British
Museum Catalogue of Snakes)
Lettering of the bones as in Fig. 3]
The principal modifications of the skull in the European genera may be
contrasted as in the following synopsis:
I. Quadrate articulating with the cranium, supratemporal absent;
mandible much shorter than the skull, with coronoid bone; maxillary
small, on lower aspect of cranium; pterygoids not extending to
quadrate; nasals forming long sutures with the premaxillary,
prefrontals, and frontal
_Typhlops._
II. Quadrate suspended from the supratemporal; mandible at least as
long as the skull; pterygoids extending to quadrate or mandible.
_A._ Mandible with coronoid bone; nasals in sutural contact with
frontals and prefrontals; transverse bone short, not projecting much
beyond cranium; maxillary not half as long as mandible, which is not
longer than skull (to occiput)
_Eryx._
[Illustration: FIG. 7--SKULL OF _Zamenis gemonensis_. (From British
Museum Catalogue of Snakes)]
_B._ No coronoid bone; nasals isolated.
1. Maxillary elongate, not movable vertically.
_a._ Maxillary half as long as mandible.
Supratemporal half as long as skull, projecting far beyond cranium;
mandible much longer than skull
_Tropidonotus._
Supratemporal not half as long as skull, projecting far beyond cranium;
mandible much longer than skull
_Zamenis._
[Illustration: FIG. 8--SKULL OF _Coluber longissimus_. (From British
Museum Catalogue of Snakes)]
Supratemporal not half as long as skull, projecting but slightly
beyond cranium; mandible much longer than skull
_Coluber._
Supratemporal not half as long as skull, not projecting beyond
cranium; mandible not longer than skull
_Coronella_, _Contia_.
_b._ Maxillary not half as long as mandible, which is longer
than skull; supratemporal not half as long as skull,
projecting beyond cranium.
[Illustration: FIG. 9--SKULL OF _Coronella austriaca_. (From British
Museum Catalogue of Snakes)]
Quadrate longer than supratemporal; maxillary much longer than
quadrate, nearly straight in front of prefrontal; a large vacuity
between the frontal bones and the basisphenoid
_Coelopeltis._
Quadrate not longer than supratemporal; maxillary little longer than
quadrate, strongly curved in front of prefrontal
_Macroprotodon._
Quadrate longer than supratemporal; maxillary little longer than
quadrate, nearly straight in front of prefrontal
_Tarbophis._
[Illustration: FIG. 10--SKULL OF _Vipera lebetina_. (From British
Museum Catalogue of Snakes)
Lettering of the bones as in Fig. 3]
2. Maxillary much abbreviated and erectile; supratemporal not half
as long as skull; mandible much longer than skull; basioccipital
with a strong process.
Maxillary bone solid
_Vipera._
Maxillary bone hollowed out
_Ancistrodon._
The vertebræ number 130 to 500--in the European forms 147 (_Vipera
ursinii_) to 330 (_Coluber leopardinus_).
The vertebral column consists of an atlas (composed of two vertebræ)
without ribs; numerous precaudal vertebræ, all of which, except the
first or first three, bear long, movable, curved ribs with a small
posterior tubercle at the base, the last of these ribs sometimes forked;
two to ten so-called "lumbar vertebræ" without ribs, but with bifurcate
transverse processes (lymphapophyses) enclosing the lymphatic vessels;
and a number of ribless caudal vertebræ with simple transverse
processes. When bifid, the ribs or transverse processes have the
branches regularly superposed.
The centra have the usual cup-and-ball articulation, with the nearly
hemispherical or transversely elliptic condyle at the back (procoelous
vertebræ), whilst the neural arch is provided with additional articular
surfaces in the form of pre- and post-zygapophyses, broad, flattened,
and overlapping, and of a pair of anterior wedge-shaped processes called
zygosphene, fitting into a pair of corresponding concavities, zygantrum,
just below the base of the neural spine. Thus the vertebræ of snakes
articulate with each other by eight joints in addition to the
cup-and-ball on the centrum, and interlock by parts reciprocally
receiving and entering one another, like the joints called
"tenon-and-mortice" in carpentry. The precaudal vertebræ have a more or
less high neural spine which, as a rare exception (_Xenopholis_), may be
expanded and plate-like above, and short or moderately long transverse
processes to which the ribs are attached by a single facet. The centra
of the anterior vertebræ emit more or less developed descending
processes, or hæmapophyses, which are sometimes continued throughout
(Fig. 11, A), as in _Tropidonotus_, _Vipera_, and _Ancistrodon_, among
European genera.
[Illustration: FIG. 11--POSTERIOR PRECAUDAL VERTEBRÆ OF _Lioheterodon_
(A) AND _Heterodon_ (B). (From British Museum Catalogue of Snakes)
_a_, Back view; _b_, lower view; _c_, side view.]
In the caudal region, elongate transverse processes take the place of
ribs, and the hæmapophyses are paired, one on each side of the hæmal
canal. In the Rattlesnakes the seven or eight last vertebræ are enlarged
and fused into one.
No snake shows any rudiments of the pectoral arch, but remains of the
pelvic are found in the _Typhlopidæ_, the _Glauconiidæ_, the _Boidæ_,
and the _Ilysiidæ_. In the first these vestiges are reduced to a single
bone (ilium?) on each side; in the second they consist of ilium, pubis,
and ischium, the latter forming a ventral symphysis, and a rudimentary
femur; whilst in the third there is a long ilium, attached to the lower
branch of the first bifurcate transverse process of the lumbar vertebræ,
bearing three short bones, the longest of which, regarded as the femur,
terminates in a claw-like spur which, in males at least, usually appears
externally on each side of the vent.
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