452 OSBURN 



case, Ciinoliosaiirns- eurymeriis, the curvature of the limb is pro- 

 duced entirely in this region, the radius being fully twice as 

 long as the ulna, but in most reptilian forms the greater length 

 of the anterior bone only compensates for other conditions. In 

 the Mosasmirs the carpus and tarsus after the manner of their 

 Lacertilian progenitors, are much wider posteriorly, especially 

 among the less modified members of the group, and the longer 

 radius and tibia help to some extent to even up this deficiency 

 in the anterior part of the limb. In the hand of Mixosaiiriis , 

 again, the longer radius compensates for the humerus, which is 

 shorter on the anterior border than on the posterior. 



Suppression of Carpus and Tarsus Toward the Posterior Side. — 

 In a few forms this is sufficient to cause a very distinct curve 

 or angulation in this region of the limb. In Delphinaptcrus, 

 GlobiocepJialus and Geosaurus the carpus is quite wedge-shaped 

 with the point posterior. In DelpJunapterus this wedge is also 

 curved. In the Ichthyosaurs and Plesiosaurs the carpals and 

 tarsals are so similar to the bones of the digits that in many 

 cases it is impossible to distinguish between them with any cer- 

 tainty. In Plesiosaurus dolicliodeirus^ however, the fifth digit is 

 seen to articulate higher up than the others, and in the very 

 advanced forms, such as Cinioliosaurus among the Plesiosaurs, 

 and Baptanodon and p ht hal j n os ata^us Sixnong the Ichthyosaurs, 

 where the pisiform (cf. Baur) has moved up alongside of the 

 epipodials to articulate with the humerus. This shifting of the 

 bones brings the digits on the posterior side of the paddle to 

 articulate higher up than the anterior ones, thus assisting in the 

 curvature of this part of the limb. In the Mososaurs as a 

 group, the carpals and tarsals are lacking or nearly so at the 

 anterior border but very well developed at the posterior side, a 

 condition inherited from their Lacertilian ancestors, according 

 to Williston. But while the less specialized Mosasaurs all 

 show this condition it is very interesting to note that in more 

 modified forms, such as Tylosaurus, the carpals and tarsals are 

 almost wanting on the posterior side and the fifth digit ap- 

 proaches articulation with the ulna. 



Curvature of the Digits. — This condition exists quite gener- 



