THE CETACEANS 1175 



those of any Ungulates, and approximate much more closely to those of Carnivores. 

 It may, therefore, be suggested that the alliance between Cetaceans and Carnivores 

 may prove to be closer than is often considered to be the case. It may be noted 

 here that the few Cetaceans inhabiting fresh water are somewhat less markedly dif- 

 ferent in structure from ordinary Mammals than are the marine representatives of 

 the order; and this leads the writer last cited to conclude that Cetaceans were first 

 modified for the assumption of a purely aquatic life from land Mammals frequent- 

 ing the banks of rivers, and that after having acquired natatorial powers in fresh 

 water, their ancestors subsequently migrated to the sea, where they have attained 

 their present remarkable development. 



Before making any further remarks on Cetaceans in general, it will, 

 cs however, be convenient to take into consideration the leading structural 

 features by which they are distinguished from other Mammals. As already men- 

 tioned, Cetaceans are characterized by their fish-like form, the head passing imper- 

 ceptibly into the body without any distinct neck, and at the other extremity the 

 trunk gradually tapering to the tail, which terminates in a forked, horizontal fin- 

 like expansion, commonly known as the " flukes." The head is large in proportion 

 to the body, with a wide mouth, often furnished with a few bristles, at least in 

 the young state. The fore-limbs are represented by flippers, incased in a continu- 

 ous skin, showing no outward indications of digits, and without the slightest trace 

 of nails or claws, while of the hind-limbs there is not the least external trace. With 

 the exception of the aforesaid sparse bristles in the neighborhood of the mouth, the 

 smooth and shining skin is entirely devoid of all traces of hair, neither has it any 

 structure corresponding to the scales of fishes. Beneath, it is underlaid by the thick 

 layer of oily fat termed ' ' blubber, ' ' the function of which is to prevent the absorp- 

 tion of the heat of the body by the water. The majority of the species have a fin on 

 the back, which is entirely composed of integument, and has no internal skeleton 

 corresponding to the similarly situated fins of fishes. The eye is extremely small; 

 and, as in fishes, the ear has no external conch, and opens by an exceedingly-small 

 aperture behind the eye. The nostrils, which may have either a single or double 

 opening, are situated on the very summit of the head, and thus reach the surface of 

 the water before any other part when the animals rise horizontally. 



In the skeleton the bones are remarkable for their loose and spongy 

 structure, and 'in the living state are saturated with oil. The majority 

 of the species have teeth, which may be confined to the lower jaw, and these repre- 

 sented only by a single pair. In all cases these teeth are of a simply conical or com- 

 pressed form; and in many of the dolphins they are much more numerous than in 

 any other Mammals. In no instance is there any replacement of the teeth; but Dr. 

 Kukenthal has discovered that in some species there are rudimentary successional 

 teeth which never come to maturity, whence it is considered that the functional 

 teeth correspond to the milk series of those Mammals in which there is a replace- 

 ment. With regard to the great number of teeth present in some Cetaceans, the 

 same investigator considers that this is due to the division of a number of trilobed 

 teeth like those of certain seals (compare the figure in Vol. II. p. 732) into three 

 parts, whereby three distinct teeth have been produced out of what was originally a 



