^54 



THE AMERICAN NATURALIST. [Vol. XXXVII. 



I. The tendency of the body to take on a "fish-like " form 

 is too well known to need any discussion, as it goes without 

 saying that such a form is best adapted to progress in the water. 

 It is most marked in such animals as are most aquatic, as the 

 Cetacea, Sirenia and Pinnipedia, and to a less extent in other 

 forms as Enhydris, Potamogale, etc. The anterior part of the 

 body tends to become more rigid and concentrated, especially in 

 the cervical region, while the posterior part of the body becomes 



In the head there is found in the Cetacea and the extinct 

 zeuglodonts a lengthening of the face with a shortening of the 



third the length of the body. In the Sirenia the face is some- 

 what elongate, but not excessiv ely so, while in the Pinnipedia it 

 is never very elongate and may be quite short, while the cranium 

 is broad and flat. The length of the head is conditioned so 

 largely by the length of the jaws that it would seem that its 

 shape is not a result merely of life in the water, so much as 



simplification of the teeth which takes place in all truly aquatic 

 mammals must also be connected with food conditions. In the 

 Mystacoceti the teeth are never functional but are present only 

 in the embryo and are absorbed before birth and replaced by 

 whalebone. As the food consists of very minute forms the 

 adaptation is evident. In the Odontoceti or toothed whales the 

 teeth are purely raptorial in character, simple and fang-like and 

 often retroverted, and admirably adapted" for the capture of the 



