ADAPTIVE MODIFICATIONS OF LIMB SKELETON 463 



portion by which the limb is attached to the girdle and by which 

 the movement of the limb is possible, though even here the 

 bone becomes much simplified, for the movements of the nata- 

 tory limb are less complex and more restricted than those of 

 the terrestrial type of limb. But, however much the propo- 

 dial element may be reduced and simplified it is always recog- 

 nizable as a propodial element. As much cannot always be 

 said in regard to the epipodials, for in the most advanced cases, 

 e. g., certain Ichthyosaurs and Plesiosaurs, these bones are not 

 distinguisable from the more distal elements except perhaps in 

 the matter of size. In such a form as Baptanodon, and in fact 

 in nearly all Ichthyosaurs above the Trias, there is an even 

 gradation from the phalanges through all the bones of the limb 

 up to and including the epipodials so that the limits of carpus 

 and tarsus cannot be distinguished either from the epipodials or 

 the digits. The metacarpals and the metatarsals are seldom 

 distinguishable in any swimming limb from the phalanges, ex- 

 cept sometimes by their larger size, and the phalanges in all 

 cases {Sirenia excepted) taper off to a point. 



The changes which take place in the epipodial region are 

 perhaps the most interesting as well as the most marked of all. 

 In the usual form of ambulatory limb the radius and tibia are 

 quite different in function and in appearance from the ulna and 

 fibula respectively, and they have quite different parts to play in 

 the formation of the joints at their proximal and distal ends. 

 In the natatory limb, with the loss of joint strain and freely 

 movable joints, the function of the two bones becomes the same 

 and they tend to take an equal part in the articulations at their 

 proximal and distal ends and to become of the same size and 

 shape. This tendency is s^^w even before the bones become 

 much reduced in length, as in the Sirenia and Mysticocete 

 whales. From this point on we have in the various groups all 

 the stages in the evolution of the paddle to the point where the 

 epipodials are merely rounded plates embedded in cartilage 

 (e. g., Baptanodoit). The toothed whales have progressed 

 farther than the whalebone whales, Globiocephahis being among 

 the most modified, but the radius is always distinguishable from 



