464 OSBURN 



the ulna. The Mosasaurs are still further advanced, especially 

 the later forms, Tylosauriis, Plioplatecarpiis, etc., and the radius 

 and ulna approach each other in appearance pretty closely. Tri- 

 assic Ichthyosaurs and most Plesiosaurs have the propodials 

 evidently differentiated from the more distal elements though 

 radius and ulna may be nearly alike in shape and size, and 

 finally, in the later members of these groups they become indis- 

 tinguishable from neighboring carpals and tarsals. 



It is not strange that Gegenbaur, casting about for an 

 incipient tetrapodial limb in connection with his ** Archi- 

 pterygium " theory, should settle upon the Ichthyosaur limb 

 as the example which should connect the fin of the fish with the 

 limb of the higher animal, or that Marsh, following the lead of 

 Gegenbaur, should hail the discovery of Baptanodon as showing 

 limbs " less specialized than those in any other known form 

 above the fishes," the humerus alone being differentiated out of 

 the " primitive cartilage." It remained for Baur to demonstrate 

 that the Ichthyosaur limb is not the least but the most special- 

 ized, approaching the condition of the fish fin as an adaptation 

 to aquatic life. 



Elongation of Digits. 



This factor in the formation of the swimming limb seems to 

 be without exception. It is one of the earliest to appear, it 

 occurs in all groups and apparently in all individuals and shows 

 no tendency to be lost. Even in such a form as Merriamia or 

 Ichthyosaurus coimmmis, where the hind limb is evidently degen- 

 erating, the digits are still elongated in comparison with the 

 remainder of the limb. In some forms all of the digits are 

 elongated, and even extra digits added to the paddles may be 

 greatly increased in length {cL the sixth digit of many Ich- 

 thyosaurs), but in other cases certain digits only are elongated 

 while the others retain their original size or even become greatly 

 reduced. Thus in the hand of GlobiocepJialus the second and 

 third digits are enormously extended while the first, fourth and 

 fifth are quite degenerate, and in Ichthyosaurus acutirostris three 

 greatly elongated digits are present while the other two, prob- 



