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.)
SIDE (A) AND UPPER (B) VIEWS OF SKULL; (C) REARWARD AND (D) FORWARD TOOTH OF YOUNG OF GANGETIC DOLPHIN. (After Gervais and Van Beneden.)
THESUSU,ORGANGETICDOLPHIN.[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 theanterior 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.
GANGETIC DOLPHIN.
FLIPPER OF 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,27–2830–32= 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.
THEINIA,ORAMAZONDOLPHIN,[232]is another of the remarkable fresh-water forms. The former name is that given to it by the Indian tribes of Bolivia. It ranges from the mouth of the river up the whole of its affluents of any magnitude, 2,000 miles from the sea. Mr. Bates, in his “Journey on the Amazon,” tells us that when it rises the top of the head is the part first seen; it then blows and immediately afterwards dips head downwards, its back curving over, exposing successively the whole dorsal ridge with its fin. It seems thus to pitch heels over head, but does not show the tail fin. It generally goes in pairs. Exceedingly numerous throughout the Amazons, it is nowhere more plentiful than in the shoaly water at the mouth of the Tocantins, especially in the dry season. The Indians have a story that the “Bouto,” as they also call this creature, “once had the habit of assuming the shape of a beautiful woman, with hair hanging loose to her heels, and walking ashore at nights in the streets of Ega, to entice the young men down to the water. If any one was so much smitten as to follow her to the water-side, she grasped her victim round the waist and plunged beneath the waves with a triumphant cry.” It is held in veneration, and on this account the Indians can hardly be induced to harpoon it. They have a superstition that blindness results from the use of its oil (which nevertheless is excellent for lamps), and though Mr. Bates prevailed upon an Indian to capture one, the fellow repented of his deed the day afterwards, declaring that his luck had there and then forsaken him. This animal is seven or eight feet long. Its colour commonly is bluish above, passing into a pale flesh-colour beneath, the tail and flippers being bluish, but the tints vary considerably, and even differ with age and season. The head is furnished with a long beak. There is a kind of keel-shaped dorsal fin, and the flippers are of fair size, broadish and tapering, thus differing from those of the Susu. The skull has a certain resemblance to that of the Gangetic Dolphin, but without the great cheek-crests peculiar to the latter, besides other minor differences. In both jaws there is a long series of stout conical teeth of a pretty uniform size. These vary in number in different specimens, as the following formulæ in two separate individuals show26–2625–27= 104;34–3233–32= 131. The muzzle of the young is hairy; while both the eye and the ear-hole are much better marked than in the Susu. It is a fish-eater, and the mother exhibits great affection and devotedness towards her young.
THEPONTOPORIA.[233]—Like Inia this is a South American form, and is now known to inhabit the mouth of the La Plata and other rivers entering into the Atlantic on the coasts of the Argentine Republic and Patagonia. But, unlike the two preceding forms, it is not confined to the rivers, for it ranges along the sea-coast. The very few specimens met with show it to be a small animal, not more than four feet long, of a blackish tint, pale beneath, with a white streak along each side from behind the blow-hole. It has an unusually long narrow beak, but not such a prominent head as in the two others. This animal has a well-marked triangular dorsal fin, and the fore-flipper is somewhat fan-shaped and broadish, and not pointed as in the Inia. The crestless skull has characters intermediate between the river Dolphins and the marine Dolphins to be described farther on. The teeth are small and very numerous, somewhat fewer in the young animal, conical in shape, with a swollen ring round their base. The dental formula is as follows:—53–5353–53= 212 or57–5754–54= 222.