THE VERTEBRAE 109 



ber, two, has remained persistent in most reptiles and even most 

 mammals to the present time. A third vertebra, from the cau- 

 dal series, was early united in many Theromorpha and the latest 

 Cotylosauria. Still another, and possibly two, were joined in the 

 Dinocephalia and Anomodontia. The Plesiosauria, purely aquatic 

 animals with propelling legs, have three or four sacrals. From one to 

 three additional vertebrae have been fused with the sacrum in front 

 in the Pterosauria (Fig. 118 d), and some Dinosauria, but they are 

 not true sacral vertebrae. 



Not only may the sacral vertebrae be closely fused, but their 

 arches and spines may become almost indistinguishably united. 

 Usually, however, the zygapophyses remain visible and are some- 

 times functional. In Iguana, even the zygosphene and zygantrum 

 are present between the two sacrals. The sacrum is lost, not only 

 in the snakes and legless lizards, but also in the mosasaurs and late 

 ichthyosaurs, where hind legs have lost locomotive functions. 



Caudal Vertebrae 



(Figs. 76, 83-85) 



The tail of the earliest known reptile, from the Coal Measures of 

 Ohio (Fig. 84), was long and slender. The Cotylosauria had, for the 

 most part, only a moderately long tail, with not more than sixty 

 vertebrae. The length of the tail, however, depends so much upon 

 habits that it may be extremely variable even in members of the 

 same order. Stumpy- tailed lizards (Trachysaurus), for instance, 

 have practically no tail, while other skinks have a very long and 

 slender one. Invariably it is long in tail-propelling, swimming rep- 

 tiles; such reptiles move sinuously through the water, preventing 

 much use of the legs as propelling organs. Those with propelling 

 legs, on the other hand, have a broader and flatter body and short 

 tail, of use only as a steering organ. However, sauropod dinosaurs, 

 though supposed to be exclusively water animals, have a very long 

 and slender tail, more or less whiplash-like at the end. As a rule, 

 swift-moving, crawling reptiles have a long and slender tail, while 

 short-tailed reptiles are invariably slow in their movements upon 

 land. 



The spines of the caudal vertebrae in land reptiles are seldom long; 

 certain chameleon lizards and the basilisc lizard are exceptions ; the 



