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. ------------------------------------------------------------------------ CHAPTER V DENTITION In the most generalized snakes--those which show the nearest approach to lizards--teeth are present not only on the rami of both jaws, but also on the premaxillary bone, on the palatines, and on the pterygoids. A reduction of the dentition takes place in various genera, in which the teeth of either the upper or the lower jaw, and of the palatines or pterygoids, or both, may be absent, and the premaxillary is devoid of teeth in the great majority, including all European representatives, of the Ophidia.