Page:The Osteology of the Reptiles.pdf/128

110 vertebrae distally are more slender and the zygapophyses weak. One of the first indications of swimming habits, at least in those reptiles 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 without 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.



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 centrum in many lizards, the Proganosauria Saphaeosaurus and Sphenodon, 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 intercentra (Fig. 76 ), occur below and between the caudal centra in