170 MAMMALIA ORDER VIILCETACEA. 



majority are thoroughly pelagic in their habits, never coming to shore 

 even for the purpose of breeding, and, in consequence, their fish-like 

 characters are even more pronounced than is the case with the sirenians. 

 The tail, or "flukes," which is always more or less distinctly forked, is, how- 

 ever, as we have seen, expanded in a horizontal, instead of in a vertical, 

 plane, and in place of the two paired fins of most fishes, cetaceans have 

 only a single pair of flippers, corresponding to the fore limbs of ordinary 

 mammals. Although these flippers are completely invested in a continuous 

 skin without any nails or external traces of digits, internally they contain 

 the same skeletal elements albeit, highly modified as their terrestrial 

 kindred. There is, however, the important exception that the bones of the 

 digits are greatly more numerous than in other mammals, although the 

 number of digits themselves never exceeds the normal five ; this peculiarity 

 offering an important distinction between cetaceans and sirenians. Like 

 fishes, many members of the present order have a back-fin ; although 

 this differs from that of the former group in lacking an internal 

 skeleton. Unlike most fishes, cetaceans have, however, a perfectly naked 

 skin ; although the general presence of a few scattered bristles, especially in 

 the young state, around the mouth, proclaims their origin from fully haired 

 mammals. And here may be mentioned the very remarkable circumstance 

 that certain extinct members of the order were furnished with a bony 

 armour, somewhat comparable to that of crocodiles ; although this does not 

 permit of our saying that all cetaceans are descended from armoured 

 ancestors. In coloration, whales and porpoises resemble most fishes 

 inhabiting the surface of the sea, such as mackerel and herrings, in that the 

 upper surface of the body is dark, and the lower light this being obviously 

 a protective resemblance, as the dark upper surface, when viewed by 

 reflected light, harmonises with the dark waves ; while the light under 

 surface, when viewed from below by transmitted light, must be ,arcely 

 distinguishable from, the bright sky above. As in the sirenians, the external 

 conchs of the ears are totally wanting, and the aperture of the ear itself is 

 extremely minute. The e es also are relatively small, and placed far back 

 on the sides of the head, which is always large in proportion to the size of the 

 body. An important difference from the sirenians is to be found in the 

 position of the nostrils, which may open externally by either a double or a 

 single aperture ; these being invariably situated on the very summit of the 

 head, and thus enabling the creatures to commence breathing the very 

 moment they reach the surface, without altering their normal horizontal 

 position. To prevent the ingress of water while beneath the surface, the 

 nostrils can be completely closed by a valve or valves. Reference has already 

 been made to the essential difference between the skeleton of the flipper of a 

 cetacean and a sirenian ; and it may be added that the vertebree of the 

 former have the terminal plates, or epiphyses, which are practically or 

 entirely wanting in the existing members of the latter group, very strongly 

 developed. The coat of fat, or blubber, lying beneath the skin, so charac- 

 teristic of all cetaceans, has been already alluded to ; but it must be also 

 mentioned that these animals have net-works retia mirabilia of blood- 

 vessels for the retention of oxygenated blood during their descent, which 

 can be used gradually, and thus enable them to stay below for a longer 

 period than would otherwise be the case. The two teats of the female differ 

 from those of the sirenians in being placed far back on the abdomen. 

 Although none of the porpoises and dolphins attain very large dimensions, 



