THE VERTEBRAE III 



most reptiles. Usually single and Y-shaped — • whence the name 

 chevron — they may be paired in the Plesiosauria and Ichthyo- 

 sauria. The medial ones of the Sauropoda have two Y-shaped, 

 broadly divergent branches united at their base. More or less ves- 

 tigial 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 ex- 

 clusivel)! to the distal part of the preceding centrum with which they 

 may be coossified, as in some mosasaurs and lizards, especially those 

 in which the cervical intercentra have migrated forward to articulate 

 or be coossified with the median hypapophysis. Chevrons primi- 

 tively, 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. 



