HABITS IN MAMMALS. 



to the food, which consists of aquatic plants. In the recently 

 extinct genus Rhytina the teeth are absent and arc replaced by 

 horny plates. In the manatee they are secondarily increased in 



occurring a great simplification of the jaws, particularly the 

 lower which tends toward the loss of all prominences for the 

 attachment of muscles, so that in the Cetacea the coronoid 

 process is often greatly reduced and the angle as well. The 

 articulation with the squamosal becomes loose and simplified, and 

 the symphysis does not ankylose except in a few cases such as 

 Platanista. Even in some of the Pinnipedia there occurs a 

 noticeable weakening of the jaw and looseness <>f the symphysis. 

 However, in the walrus whose food consists chiefly of bivalve 

 molluscs, the teeth are adapted to crushing the shells and the 

 jaw is remarkably heavy and strong and in the adult the sym- 

 physis is thoroughly ankylosed. The crushing jaw of the 

 Sirenia is also ankylosed. In general the pterygoid processes 

 also tend to become reduced, and there is a tendency toward a 

 looseness of articulation in all the bones of the head, particu- 

 larly in the Cetacea. 



The shifting of the external nares from a terminal into a more 

 dorsal position is an adaptation to breathing at the surface of the 

 water. This is accomplished by a shortening up of the nasal 

 bones, which in the Cetacea become merely vestiges on the 

 anterior surface of the frontals. In the Cetacea the opening is 

 so shifted as to lie quite on top of the head, while in the Sirenia 

 and Pinnipedia also the shitting is quite noticeable. It is a note- 

 worthy fact that the true seals. Phocickc. and the dugong, Halicore, 

 which give other indications of a longer life in the water than 



