460 OSBURN 



of the Ichthyosaurs, the simility in shape, size and angle of at- 

 tachment is easily noted. The Thalattosuchia have the hmbs 

 so greatly different in length that it is difficult to find much 

 similarity except in the matter of curvature and angle of 

 attachment. 



It is to be noted that in the matter of curvature or backward 

 extension the limbs are always similiar — if the fore limb is 

 straight the hind limb is also found to be straight, but if the 

 fore limb is curved the hind limb will assume about the same 

 degree of curvature. (Cf. Mixosaiiriis, Toretocnemus^ Platecar- 

 pus, Tylosaurus^ Plesiosaurus, Dolicliorynchops^ etc.). 



The Tendency Toward the Loss of the Hind Limbs. 



This character in those forms which have developed a caudal 

 fin is found almost without exception. In such forms the tail 

 becomes a much more efficient propelling organ than the limbs 

 and the latter are of use chiefly as organs of equilibration. The 

 anterior limbs, from their position, are naturally more useful in 

 this respect than the posterior and the latter tend toward reduc- 

 tion. In aquatic animals which have not developed a tail fin 

 we have two widely different conditions. The Pinnipedia illus- 

 trate one condition where the hind limbs swing backward and 

 together act as a propeller very much after the fashion of the 

 flukes of a whale — so much so in fact, as to have suggested to 

 Ryder the mistaken hypothesis that the flukes of the Cetacean 

 are only misplaced and modified hind limbs. The long and 

 supple vertebral column renders this motion possible in the 

 Pinnipedia. In the Chelonia the nature of the spinal column 

 and the box -like exoskeleton prevent any such use of the hind 

 legs as is seen in the seals, and the fore legs become greatly 

 enlarged organs of propulsion while the hind paddles are com- 

 paratively very weak. The skeleton of TlialassocJielys, for 

 example, reminds one of a side-wheel steamboat with enor- 

 mously large paddles and with twin rudders behind. 



Among the forms with a tail fin the Cetacea and Sirenia are 

 preeminent with the entire loss of a posterior limb externally. 

 The Ichthyosauria at first seem a little puzzling from the fact 



