ADAPTIVE MODIFICATIONS OF LIMB SKELETON 459 



partial retention of terrestrial habits probably accounts for both. 

 The Nothosauridae, probable ancestors of the Plesiosaurs, while 

 undoubtedly aquatic, had not necessarily — judging from the 

 condition of the limbs — lost their land connection, and they 

 show but little distal dilatation of the bones. Lariosaurus has 

 the humerus well curved and somewhat expanded distally, and 

 Shnosaurus, according to the plates in Von Meyer's " Fauna der 

 Vorwelt," shows considerable distal enlargement. If the No-- 

 thosauridse are the ancestors of the Plesiosauridce, as seems to 

 be the case, we have in these two families the most complete 

 story of the evolution of the swimming Hmb known in any one 

 group, for the earliest Triassic members of the Nothosauridae 

 are semiaquatic while the latest Cretaceous forms of the Plesio- 

 sauridae are thoroughly adapted for marine life as von Zittel 

 has already pointed out (1902, p. 172). 



Parallelism of Fore and Hind Limbs. 



In many aquatic forms there is a very evident parallelism 

 between the fore and hind limbs, and a correspondence in action 

 which is never met with in terrestrial forms. The fore and 

 hind limbs in most cases meet the water in the same manner 

 with the result that they become very similiar in structure. 

 The Pinnipedia are an exception, for, while both fore and hind- 

 limbs are evidently adapted to aquatic life, there is a very 

 different use made of them in swimming. Cetacea offer no 

 evidence on either side as the limbs of ancestral forms are 

 not known and recent forms lack the posterior limbs. A com- 

 parison of the fore and hind Hmbs in Ichthyosauria, Plesiosauria 

 or Mosasauria, however, shows great similarity in shape and 

 structure as well as in their angle of attachment with the body. 

 There can be no question but that the fore and hind paddle of 

 a Plesiosaur acted in the same manner in propelling the body 

 through the water. In Mosasaurs the same thing is evident. 

 In later Ichthyosaurs, owing to the development of the tail fin, 

 the hind limb becomes much reduced, but still it retains its 

 similarity to the fore limb (cf. Ichthyosaurus communis, I. Quad- 

 riscissus etc.), while in Mixosaurus, which is the least modified 



