WILLISTON : AQUATIC AIR-BREATHING VERTEBRATES. 261 



example of such is the dolphin of the Ganges, a fish-eating 

 cetacean. This type of head is best illustrated in the gar-pike, 

 a surface-feeding fish, whose food consists almost exclusively 

 of small fishes, which are caught by quick movements of the 

 body and jaws rather than by rapid swimming. I doubt very 

 much whether any of the reptiles before mentioned had the 

 ability to move with any great degree of speed through the 

 water. 



In the long-necked plesiosaurs the head is broader, Hatter, 

 and less elongate, with the teeth larger and more unequal in size. 

 In the mosasaurs the skull is larger, less elongate, though still 

 of a distinctly elongate type, especially in the more specialized 

 forms, such as Tylosaurus. The teeth are larger, are less firmly 

 implanted in the jaws, and there are additional teeth upon the 

 pterygoids, wanting in the other aquatic vertebrates. Further- 

 more, the mandibles have a peculiar lateral articulation ; all of 

 which characters indicate larger and more powerful prey. In- 

 deed, remains of fishes of considerable size have been found in 

 the fossilized stomach contents of these animals. 



Plesiosaurs, or at least some of the long-necked forms, had 

 the strange habit of swallowing siliceous pebbles, sometimes of 

 considerable size and in considerable quantities ; as many as a 

 peck have been found among the remains of a single animal. 

 Seals and crocodiles are said to have similar habits ; the reason 

 therefor is hard to conjecture. These pebbles, moreover, were 

 carried in the plesiosaur's stomach until reduced to very small 

 dimensions, indicating clearly the smallness of the pyloric 

 orifice and the complete digestion of the food material. Copro- 

 lites, evidently of mosasaurs, are frequently found, with large, 

 undigested fish bones, and fragments of fish bones, and sepia 

 shells have been found in reputed ichthyosaurian coprolites. 



The posterior position of the external nares, not only in the 

 cetaceans, but also in the ichthyosaurs, plesiosaurs and para- 

 suchian "crocodiles," is doubtless due to environmental causes. 

 In the mosasaurs the nares are large, but are located anteriorly, 

 though not at the extremity of the snout. Why they should 

 be so small and situated so far back in the ichthyosaurs, plesio- 

 saurs and Belodon is not evident. 



In all the aquatic reptiles sclerotic plates are well developed. 

 They are present in the plesiosaurs, a fact hitherto published, 



I7-Kan. Univ. Sci. Bull., No. 9, Vol. I. 



