462 OSBURN 



The most highly adapted Mosasaurs possessed a tail fin pro- 

 duced by the elongation of the neural and haemal spines and 

 while this was probably not as effective a propelling organ as 

 the more highly specialized flukes of the whales and Ichthyo- 

 saurs, yet its presence was sufficient to cause a reduction in the 

 hind limbs. Williston has already remarked upon this evident 

 " weakening of the hind limbs, particularly in the more special- 

 ized groups like the Tylosaurinae." In Clidastes velox the 

 hind limbs seem much weaker than the anterior ones. 



The Thalattosuchia seem to be an exception to the rule, for 

 according to Fraas, Geosaurus suevicus, the only thoroughly 

 known form of the group, has the paradoxical combination of a 

 well developed tail fin and a hind limb very much larger than 

 the fore limb. The fact that this limb shows only compara- 

 tively little adaptation to aquatic life suggests the possibility 

 that Geosmirus had only recently lost its land relation, the well 

 developed tail fin notwithstanding. 



Tendency Toward Similarity in Shape and Function of 

 ALL THE Bones of the Limb. 



In the limbs of swimming animals there is no occasion for 

 the specialization of various parts of the skeleton such as ex- 

 ists in terrestial animals. All the elements of the limb skeleton 

 except the propodial have approximately the same function to 

 perform and consequently all parts except the humerus and 

 femur tend to become very much alike, and even the distal ends 

 of these bones may lose their differentiated character and become 

 mere rounded plates. The process is one of degeneration in 

 which all the differentiation brought about by the varied stress of 

 terrestrial life is lost and the bones become at last in the most ex- 

 treme cases mere rounded plates — ** Stutzplatten " — with no 

 characteristics by which one can be distinguished from another. 

 These may be either closely set, " eine Pflaster von polygonalen 

 Flatten " (Fraas), as in most Ichthyosaurs, or separated by carti- 

 lage in which they are embedded, as in Baptanodon and Ichthyo- 

 saurus iJtgens. The propodial element retains in part at its proxi- 

 mal end its original function, — at least it continues to be the 



