WILLISTON : AQUATIC AIR-BREATHING VERTEBRATES. 263 



Ichthyosaurus and Gcosaurus the downward curvature of the 

 distal caudal vertebrae gave support to the lower side of the 

 caudal fin. In the mosasurs the tail is not decurved, and the 

 dilatation was evidently not terminal but along the distal part. 

 In the plesiosaurs the tail is never greatly elongated, and in 

 some was probably short. There was, however, a terminal, fin- 

 like dilatation, and, in some forms at least, the distal caudal 

 vertebrae were curved upward. 



The tendency toward a metamorphosis of the ambulatory into 

 natatory limbs is constant ; it is always more pronounced in the 

 anterior than in the posterior extremities. In the cetaceans and 

 sirenians the hind limbs have disappeared entirely in the living 

 forms, though the earlier, extinct sirenians were also provided 

 with hind extremities. In the ichthyosaurs the hind limbs 

 are always decidedly smaller than are the front ones, while in 

 the mosasaurs a similar, though less pronounced, weakening of 

 the hind limbs is evident, particularly in the more specialized 

 groups like the Tylosaurimr. Among the plesiosaurs the hind 

 extremities are always powerful and well developed, though in 

 all the forms known to me they are less powerful than the front 

 ones. Here the relative shortness of the tail has imposed a dis- 

 tinct propelling or sculling function upon the hind limbs, as is 

 the case to a much greater degree among some of the pinnipeds. 

 In Geosaurus, on the contrary, the hind limbs are larger than 

 the fore limbs, while the tail is elongated and provided with a 

 terminal fin, evidently of no small size. But these legs in 

 Geosaurus were distinctly ambulatory and of little use in the 

 water ; they were doubtless carried close to the tail in swim- 

 ming, as in the partly metamorphosed tadpole. 



Hyperphalangism is best exhibited in the cetaceans, ichthyo- 

 sauria, and plesiosauria ; and in the ichthyosaurs there is hy- 

 perdactylism also. With our present knowledge, it would seem 

 remarkable that serious argument was needed to controvert 

 Gegenbaur's views as to the ichthyic affinities of the paddles in 

 these animals. Among the mosasaurs hyperphalangism is more 

 pronounced in the more specialized Tylosaurus than in the other 

 groups. While in Mosasaurus the fifth digit has been lost in the 

 hind foot, and in other forms is reduced in size, inherited terres- 

 trial characters, in Tylosaurus the fifth toe has become greatly 



