64 TOOTHED WHALES AND THEIR ANCESTRY. 



great height of their nearly vertical back-fins. Allied 

 cetaceans with smaller teeth [Orcella) frequent the Bay of 

 Bengal and ascend some distance up the Irrawadi. Another 

 well-marked type is the black-fish (Fig. 23), characterized 

 by its remarkably short and rounded head, the uniformly 

 black hue of the skin, and by the eight or twelve small 

 and conical teeth being confined to the front portion of the 

 jaws. 



There are other less well-known representatives of this 

 group which we have not space to notice ; and we 

 accordingly pass on to say a few words about dolphins — a 

 term which should be restricted to those forms having a 

 distinctly marked beak. Since, however, sailors will 

 persist in speaking of dolphins indifferently, either as 

 bottle-noses or porpoises, the inexperienced landsman 

 must be on his guard not to confound them when 

 thus spoken of either with the bottle-nosed w bales 

 or the true porpoises. Dolphins, which are divided into 

 numerous genera, according to the number of their teeth, 

 the length of the beak, and other characters, are all 

 comparatively small species, seldom exceeding some ten 

 feet in length ; and while the great majority are marine, a 

 few ascend some of the larger tropical rivers, such as the 

 Amazon. Fish of various kinds constitute their usual 

 prey ; but one peculiar species recently described from the 

 Cameruns district is believed to subsist on sea-weed. Of 

 the better-known tj'pes, the common dolphin represents 

 the genus Delphinns, the bottle-nosed dolphins constitute 

 a distinct genus (rwcs/o^w), while the long-beaked dolphins 

 are separated as Steno. 



To the foregoing group of beaked dolphins the ordi- 

 nary observer would, doubtless, be disposed to refer three 

 peculiar species severally restricted to the larger rivers of 

 India, the Amazon, and the mouth of the La Plata river, 

 but as these differ more or less markedly from other 

 dolphins in certain structural features they are referred to 

 a distinct family. Moreover, since these peculiarities 

 approximate to a more generalized type, while their fresh- 

 water habits and scattered distribution indicate extreme 

 antiquity, it is not improbable that these three dolphins 



