ADAPTIVE MODIFICATIONS OF LIMB SKELETON 457 



the short humerus. Even in the Sirenia there is considerable 

 broadening in this region. Among the reptiHan groups the 

 epipodials are greatly modified in every case examined, except 

 in the hind limb of Thalattosuchia. In a number of cases the 

 greatest breadth of the limb is found in this portion. In Plesi- 

 osaurs this seems especially to be true. In Cimoliosaurus tro- 

 chanterus and C. portlandicus the position of the ''pisiform'' 

 alongside of the ulna and its homologue beside the fibula makes 

 this region very evidently the widest. In the hand of the 

 Thalattosuchian Geosaiinis the breadth is greatest in this part, 

 due to the extreme breadth of the very short radius and ulna. 

 In Mosasaurs also these bones are very broad, and in every 

 case in Ichthyosaurs they are much broadened though this re- 

 gion is usually not as broad as the more distal portion of the 

 limb. 



There are two factors producing the breadth in the epipodial 

 region, the increase in the breadth of the bones themselves, and 

 the separation of the bones at their distal ends. In the Mam- 

 malia there is no real separation of the distal ends of the radius 

 and ulna though in some cases (cf. Globiocephalus) the inter- 

 medium interlocks between their ends. In Plesiosauria and 

 Thalattosuchia also no distal separation is evident, and in Ich- 

 thyosaurs it is seen only in Mixosaurus where it is quite notice- 

 able and is probably a primitive character. In the Mosasaurs, 

 however, they may be separated at a very wide angle (cf. espe- 

 cially the hands of Clidastes and Platecarpits) and all show it to 

 a greater or less extent. It is possible that this tendency has 

 been derived directly from their Lacertilian ancestors which 

 show some distal separation, but, however its presence is to be 

 accounted for, it seems pretty certain that the Mosasaurs have 

 developed it greatly and made the most of its possibilities in 

 broadening the paddle. Stereosterniim, one of the Rhyncho- 

 cephalia, shows quite a little distal separation of the epipodials. 

 It also has the fifth digit set out at an angle as in the Mosa- 

 saurs. 



The humerus and the femur are probably the last bones of 

 the limb to become affected by distal dilatation, but in the most 



