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. 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 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, Fig. 4—Skull of Typhlops lumbricalis. (From British Museum Catalogue of Snakes) Lettering of the bones as in Fig. 3 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. 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. Eryx. 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 Tropidonotus. Supratemporal not half as long as skull, projecting far beyond cranium; mandible much longer than skull Zamenis. 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. 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. Tarbophis. 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. 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” 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. |