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 jointery. 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.