94 



THE OSTEOLOGY OF THE REPTILES 



and certain pleurodiral turtles have saddle-shaped articulations. In 

 no other order of reptiles are the variations so great as in this. 



The pleurocoelous (Fig. 75) presacral vertebrae of the sauropod 

 dinosaurs are peculiar in having large cavities in the centra, sepa- 

 rated by a median partition, with an oval or round orifice at each 



side. Not only is the centrum thus light- 

 ened in these dinosaurs, but the arch is 

 curiously strengthened by plates and but- 

 tresses. Certain other South African rep- 

 tiles {Tamboeria) also have pleurocoelous 

 vertebrae. It is supposed that this hollow- 

 ness and lightness of the cervical and 

 dorsal vertebrae, correlated with the other- 

 wise solid or cancellous skeleton, served 

 to keep the body erect in water, their 

 natural habitat in wading or swimming. 



Except in the snakes and legless lizards, 

 where but two regions are recognized, the 

 caudal and precaudal, the spinal column 

 of reptiles is divisible into cervical, dorsal, 

 sacral, and caudal regions, and sometimes 

 lumbar also, as in mammals. The cervical 

 region is that in front of the shoulder- 

 girdle, the dorsal that between the shoulder 

 and hip girdles, the sacral that which sup- 

 ports the hip girdle, and the caudal that of 

 the tail. 



We may hardly venture to guess as to 

 the primitive number of vertebrae in rep- 

 tiles. We are quite sure that there has 

 been an increase in number in some, a 

 decrease in others. The land temnospondylous amphibians that 

 we know have but one real cervical vertebra, the so-called atlas, 

 twenty-two to twenty-five dorsals, one or two sacrals, and a short or 

 moderately long tail. Trimerorhachis, an aquatic Lower Permian 

 temnospondyl, has thirty-one precaudal vertebrae and no differen- 

 tiated sacrals. The earliest reptile that we know, Eosauravus, 

 subaquatic in habit, had at least twenty-four or twenty-five pre- 



FiG. 75. Dorsal vertebra of 

 Diplodocus (Saurischia). After 

 Hatcher. One tenth natural 

 size. 



