264 KANSAS UNIVERSITY SCIENCE BULLETIN. 



elongated, and its phalanges increased in number commensurate 

 with those of the other digits. 



With hyperphalangism has occurred also a shortening of the 

 limb bones, especially those of the front pair, which are the first 

 to show natatorial characters. There has also occurred a re- 

 striction of articular movement, with imperfect joints. Those 

 ichthyosaurs (Mi.vosaurus) and plesiosaurs (Plesiosaurus) which 

 have preserved an elongated form of the epipodial bones are 

 clearly more generalized than those in which the bones have 

 become mere polygonal platelets, a contestation long ago made 

 by Baur in support of the terrestrial derivation of the ichthyo- 

 saurs. Concurrent with such changes is the presence often of 

 three or even four elements in the epipodial series. The at- 

 tempt to correlate these with mesopodial elements of the ter- 

 restrial vertebrate is, I believe, futile. Accessory, sesamoid, 

 mesopodial, or whatever they may be called, they are in reality 

 new growths, like the sixth and seventh digits of the ichthyosaur 

 paddle, or the supernumerary digits of the human hand or foot, 

 and not misplaced carpals and tarsals. It would not surprise 

 me greatly to learn of plesiosaurs with additional digits. 



In Geosaurus there is a distinct beginning of the aquatic pad- 

 dle in the fore limb, although there is scarcely any change from 

 the ambulatory limb behind. As an example of a stage through 

 which probably all other aquatic vertebrates have passed, the 

 thalattosuchia are very instructive. In those aquatic lizards, 

 the mosasauria, we find a more advanced stage, with shortening 

 of the bones and hyperphalangism more or less pronounced, but 

 the widely spreading digits, the relatively large size of the hind 

 limbs, the presence of a sternum in the more generalized forms 

 at least, and the less elongated head, are all indications of a 

 lesser specialized stage than is seen in the cetaceans, ichthyo- 

 sauria, and plesiosauria. 



In the cetacea and sirenia, the sternum, though reduced, is 

 not entirely lost, nor is it lost in the mosasaurs, at least in the 

 more generalized groups. It has not suffered in the aquatic 

 crocodiles, but in the ichthyosaurs and plesiosaurs it has van- 

 ished utterly. The sacrum is wanting in all the true aquatic 

 types. No longer needed for the support of the body in am- 

 bulation, the united vertebrae, with rigid or close union with 

 the ilium, must have interfered with free movements of the 



