text: The centra have the usual ball and socket joint, with the nearly hemispherical or transversely elliptic condyle at the back (procoelous vertebrae), while 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 vertebrae 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 mortise and tenon joints. The precaudal vertebrae 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 vertebrae emit more or less developed descending processes, or haemapophyses, which are sometimes continued throughout, as in Tropidonotus, Vipera, and Ancistrodon, among European genera.
  In the caudal region, elongate transverse processes take the place of ribs, and the haemapophyses are paired, one on each side of the haemal canal. In the rattlesnakes the seven or eight last vertebrae are enlarged and fused into one.
  Vestigial limbs
  No living snake shows any remains of the pectoral arch, but remains of the pelvis are found in:
  Boas and Pythons: a long ilium, attached to the lower branch of the first bifurcate transverse process of the lumbar vertebrae, bearing three short bones, the longest of which, regarded as the femur, terminates in a claw-like pelvic spur which usually appears externally on each side of the cloaca.
  Leptotyphlopidae: ilium, pubis, and ischium, and rudimentary femur, the ischium forming a ventral symphysis.
  Aniliidae
  Typhlopidae: a single bone on each side.