THE RIVER DOLPHINS (PLATANISTIDÆ).
Three living forms come under this heading, which, however, barely present such characters in common as to render them a compact group; and some authorities even incline to regard them as representative of sub-families. As in the Seal-toothed Whales their neck vertebra are separate.
SIDE (A) AND UPPER (B) VIEWS OF SKULL; (C) REARWARD AND (D) FORWARD TOOTH OF YOUNG OF GANGETIC DOLPHIN. (After Gervais and Van Beneden.)
THE SUSU, OR GANGETIC DOLPHIN.[231]—This remarkable Cetacean is never found in the salt water, or at best only in the brackish water of the Sunderbunds; its habitat being the rivers Ganges and Indus from their mouths upwards, and their various tributaries almost to the mountain ranges in the north. Specimens have been got at least 1,000 miles beyond Calcutta. It measures from six to twelve feet in length, and in colour is entirely sooty black. Its long body has a moderate girth, and just behind the middle of the back there is a slight elevation which can barely be called a fin. The tail is broadish; the flippers are short, very broad, fan-shaped, and not pointed as in most Whales. The head is globular, with a long, narrow, spoon-shaped snout. The opening of the blow-hole, unlike that of other Whales, excepting the Inia, is not transverse, but a single longitudinal slit. The eye externally, situated above the angle of the mouth, is so diminutive as barely to be visible. We may compare the Susu to the Mole in this respect, for in an adult eight feet long the whole of the eyeball is no bigger than a pea in size. Small though this eye is, nevertheless it is perfect in lens and humours, &c. The ear-orifice behind the latter may be compared to a pin-hole. The narrow rostrum of the upper and of the lower jaw is implanted with a series of teeth, more pointed and conical in front, and narrower and laterally flattened in those behind. In the young animal the difference between the anterior and the posterior teeth is exceedingly marked in size, the former being very long, the latter very short, while as age advances quite the reverse is the case. The back teeth also wear down very considerably in the crown, and increase in breadth in root-substance; indeed, as Dr. J. Anderson has shown, the true dental material is worn away, and finally nothing but bone is left. The head of the male is about two-thirds the length of that of the female, and in both its point is slightly upturned. The apparently rounded skull behind the snout has broad thick zygomatic arches, and above and in front of these the cheek-bones (maxillæ) each send forwards and inwards a great roughened sheet of bone or crest, which forms a kind of open helmet. In the large hollow between these bony plates, and somewhat behind, are situated the nasal orifices, which are slightly awry.
GANGETIC DOLPHIN.
FLIPPER OF GANGETIC DOLPHIN.
The Susu frequents the deep reaches and creeks of the river, occasionally coming to the surface to blow, and although often heard are but seldom captured. Ordinarily their movements are slow, but at times they seem exceedingly active. Their food is chiefly fish, shrimps, &c., which they grovel for among the mud, something like Pigs wallowing in the mire. Grass, rice, and shells have been found in their stomachs, but Dr. Anderson has clearly shown that they are not vegetable feeders, for in the rainy season, when great tracts of land are under water, these animals pursue the fish right into the submerged “paddy-fields,” and the grass is thus most probably swallowed with their prey. The Hindoos have religious superstitions concerning the Susu. It certainly is one of the oldest known Cetaceans, since Pliny and Ælian both allude to it. It has been supposed that the kind which inhabits the Indus was a separate species, but this error has doubtless arisen from the great difference in size of the skulls of the two sexes. This animal must be all but blind, the optic nerve being no thicker than a thread; but the fact of its living habitually in muddy water renders sight less necessary than it otherwise might be. Its peculiar dentition, so like that of the ancient Squalodons in many respects, is of exceeding interest. The following is the dental formula of one specimen, 27–28 30–32 = 117. The broad roots of the rearmost teeth are usually grooved, and this gives them a deceptive appearance of possessing more than one fang; moreover, differing as the teeth do front and rearwards, still distinctions as to incisors, canines, and molars can hardly be said to exist.