i6 



S. W. WILLIS TON 



Just what has been the cause in each case of the change from 

 amphicoelous to procoelous or opisthoccelous vertebrae may not be 

 apparent, but it is no more difficuU to believe that this change 

 occurred in the different phyla of crocodiles than in the different 

 orders of reptiles. We know that all procoelian reptiles cannot l)e 

 traced back to a common procoehan ancestry. The earliest verte- 



FiG. 12. — Coelosuchus reedii. Posterior portion of left mandible, one-eighth natural sizs 



bra; of this type were apparently those of the Rhaetic or Liassic 

 pterodactyls. Many of the squamata had changed from amphicoelian 

 to procoelian by the close of the Jurassic times, and it is not at all 

 improbable that the beginning of the change in the crocodiles 

 occurred about this time; certainly so, if the vertebras described by 

 Seeley from the Wealden are really those of crocodiles, as seems 

 probable. 



The geckos among the lizards, chiefly climbing or pendent 

 animals in habit, have retained amphicoelous vertebras, long after 



