Ii8 THE OSTEOLOGY OF THE REPTILES 



arch. Real sacral ribs have been in all cases added from behind, since 

 the caudal ribs have retained more or less in all animals their original 

 attachments, while the lumbar or posterior dorsal ribs have often 

 undergone changes. It is improbable that there has ever been any 

 ''migration" of the sacral vertebrae; that is, the first true sacral 

 vertebrae of all animals^ are identical with the single sacral vertebra 

 of Seymouria. Additional sacral ribs have been due to the gradual 

 elongation of the basal caudal ribs and their articulation with the 

 ilium, as shown in the tail of the alligator snapper turtle. The second 

 and third pairs were added very early in the history of reptiles. 



Remarkably, in the Lacertilia evidences of sacral ribs have not 

 been found, the ilia being supported by transverse processes, out- 

 growths of the centra (Moodie). 



Not only are the two sacral ribs of the Crocodilia (Fig. 121) 

 primitive in their attachments, but the centra also have retained 

 their primitively amphicoelous structure. 



On the other hand, additional vertebrae have joined the sacrum in 

 front, as many as three in some reptiles, but in such cases the ribs 

 have not reverted to their primitive attachments if modified, though 

 they may extend to the ilium. In the Ceratopsia three lumbar verte- 

 brae have been fused with the sacrum, and their diapophyses with 

 the ilium. Indeed in some instances {Monoclonius for instance) a 

 vestigial free rib may remain on the first, so-called sacral vertebra. 

 In the later pterodactyls there are several such sacro-lumhar verte- 

 brae, and also in the Anomodontia (Fig. 119 c), groups that have 

 been accredited with from seven to ten sacral vertebrae. In all these 

 the ilium is greatly prolonged in front of the acetabulum. The pro- 

 jections from the vertebrae have often been called indiscriminately 

 transverse processes, but that term is true only of the sacro-lumbars. 



Whether or not the dinosaurs acquired the third or the fourth 

 sacral vertebrae after their divergence from their immediate ances- 

 tral stock is perhaps a question. But two are accredited to Hallopus, 

 a primitive type. There can be no question, however, but that the 

 dinosaurs, both the Saurischia and the Ornithischia, descended from 

 reptiles with but two sacral vertebrae, since the allied Crocodilia 



^ [But few contemporary morphologists would endorse this view. It certainly does 

 not apply to the Amphibia and is very doubtful for the Chelonia. — Ed.] 



