AQUATIC MAMMALS 



SiRENiA (all) Cetacea (all) 



Trichechidae Archaeoceti (extinct) 



Halicoridae Odontoceti 



Hydrodamalidae (extinct) Mysticeti 



Perhaps the tapir should not be included in the above list, while 

 there are doubtless those who will consider that in it should be placed 

 other mammals, such as other swamp-loving ungulates. 



No attempt will be made to present a complete diagnosis of the 

 families and genera included, but only a brief characterization and the 

 salient points which are likely of significance from the viewpoint of 

 their aquatic specialization. Preceding these, in order to give the general 

 reader a better understanding of the chapters to follow, will be a general 

 consideration of the mammals and their habits. The precise derivation 

 of different aquatic mammals is not here of particular concern, but it 

 seems advisable to touch briefly, without any great array of supporting 

 evidence, upon the probable stock from which the more specialized of 

 aquatic mammals were derived, in order that we may more intelligently 

 follow the probable course of their development. 



It is virtually certain that even in those mammals only slightly aquatic 

 there are present valvular mechanisms for the closure of the nostrils 

 and external ears, and these two points will receive no further mention in 

 the present chapter. 



MONOTREMATA 



Ormthorhynchus — platypus or duck-bill: an inhabitant of quiet 

 streams and rivers of Australia and Tasmania. It feeds upon worms 

 and similar food upon the stream bottoms. It constructs burrows for 

 resting and for raising its young, and is capable of traveling over land 

 somewhat clumsily but at considerable speed if its stream should go dry. 

 The premaxillaries and maxillaries are expanded anteriorly and support 

 a naked beak which superficially resembles that of a duck, and which 

 is covered with a soft and very sensitive membrane. The body is 

 flattened horizontally and covered with a fine, dense underfur, beyond 

 which projects a coat of coarser hairs. The tail is rather short, very 

 broad and compressed. The eyes are very small and there is no ex- 

 ternal pinna to the ear. The legs are short and well modified for 

 swimming, the toes being webbed and with large claws. The webbing 

 of the forefoot extends well beyond the claws, and this anterior part 

 is folded back when the animal progresses upon land, is digging, or 

 combing its fur. In swimming the forelimbs are used almost exclu- 



[25} 



