no 



THE OSTEOLOGY OF THE REPTH^ES 



vertebrae distally are more slender and the zygapophyses weak. One 

 of the first indications of swimming habits, at least in those rep- 

 tiles with long tails, is the widening and elongation of the caudal 

 spines throughout, [less] at first [anteriorly] and then more distally 

 until a terminal fin is developed with the end of the column in 

 the lower lobe (Fig. 85). 



The basal caudal vertebrae, from one to six in number, those with- 

 out chevrons but with ribs, are called pygals. They have the ordinary 

 intercentra in those reptiles in which they [intercentra] are persistent 

 throughout; sometimes with rudimentary chevron-like processes. 



Fig. 85. Tail, scapula (sc), and coracoid {cor) oi Geosaitrus (Thalattosuchia). After Fraas. 



The cloaca in the living animal occupies the space below them. The 

 number is more or less reduced in modern reptiles; the Crocodilia 

 have but one, most lizards, two. 



There is an unossified vertical septum through each caudal cen- 

 trum in many lizards, the Proganosauria Saphaeosaurus and Spheno- 

 don, along which it readily breaks, causing the easy loss of the distal 

 part. This septum was once supposed to represent the division 

 between the primitive component parts of the centrum. It is now 

 thought to be an acquired character, not occurring in the early 

 embryo. 



Chevrons, or haemapophyses (Fig. 84) for the protection of the 

 vessels on the under side of the tail, really outgrowths from the inter- 

 centra (Fig. 76 d), occur below and between the caudal centra in 



