Page:The Osteology of the Reptiles.pdf/129

Rh most reptiles. Usually single and Y-shaped—whence the name chevron—they may be paired in the Plesiosauria and Ichthyosauria. The medial ones of the Sauropoda have two Y-shaped, broadly divergent branches united at their base. More or less vestigial in the turtles, they are absent in snakes, replaced by a pair of vertical hypapophysial-like processes (lymphapophyses).

Chevrons articulate as a rule intercentrally, but sometimes exclusively to the distal part of the preceding centrum with which they may be coössified, as in some mosasaurs and lizards, especially those in which the cervical intercentra have migrated forward to articulate or be coössified with the median hypapophysis. Chevrons primitively, as in the temnospondyl amphibians, have their branches united above in an intercentrum-like bone, a condition found in the proximal chevrons of Sphenodon. In later reptiles, for the most part, the two branches articulate separately. At the tip of the tail they are vestigial or absent.