The generic characteristics of these dolphins are, according to Dr. Anderson, as follows: "Head globular; dorsal fin low, situated behind the middle of the body; pectoral fins oval, about one-sixth the length of the animal; teeth conical, large, and fewer in the lower than in the upper jaw, thirteen to seventeen teeth in the upper and twelve to fourteen teeth in the lower jaw; skull beaked; beak broad at the base, anteriorly pointed; premaxillary not much laterally dilated, bearing one tooth; vertebræ sixty-two to sixty-three; first two cervical vertebræ ankylosed; lumbar transverse process moderately long; vertebræ ribs twelve to thirteen, with one or two free ribs; pelvic bones opposite thirty-fifth and thirty-sixth vertebræ."
These are the dolphins which were procured by Mr. Blyth in the Hooghly, and were supposed by him to be the young of the ca'ing whale (Globicephalus), which idea has also been adopted by Jerdon; but it has been since proved that the skeletons prepared from these supposed young whales are those of adults fully matured, and not of young animals, which have certain resemblances to Globicephalus as well as to the killer whales, Orca, from which the generic name has been derived, but yet was undoubtedly distinct. The killer whales have a very high dorsal fin in the middle of the back, with very large pectoral flippers as broad as long; in Orcella the back fin is low and behind the middle of the body, and the pectoral fin is only half as broad as long. In the ca'ing whale the back fin is more towards the shoulders, and the flippers are long and narrow; the genus Orcella in fact seems to be intermediate between the dolphin and the ca'ing whale, combining the head of Globicephalus with the body of Delphinus. Dr. Anderson, however, points out further differences than the external ones I have above alluded to. Orca, he says, is distinguished by a "more powerfully built skeleton, with considerably fewer vertebræ, there being only a maximum of fifty-three in it to a maximum of sixty-three in Orcella." In Orca generally four or five cervical vertebræ are ankylosed as in the cachelots, but in the two species of Orcella only the atlas and axis are joined. "In the killers and ca'ing whales the ribs are transferred to the transverse processes at the seventh dorsal, whilst in Orcella the transference does not take place until the eighth." The skull resembles that of Orca in the breadth of the upper jaw being produced by the maxillaries, whereas in Globicephalus this effect is caused by the premaxillaries. The teeth resemble the killer's.
As I have said so much about the killer whale, I may digress a little to explain what it is, though it is not a denizen of the Indian seas. It is to the Cetacea what the shark is to fishes—a voracious tyrant with a capacious mouth, armed with formidable teeth. It hesitates not to attack the largest sperm and Greenland whales, and the smaller whales, porpoises and seals will spring out of water and strand themselves on shore in terror at its approach. It ranges from twenty to thirty feet in length, and is of so gluttonous a character that in one recorded case a killer had been found choked in the attempt to swallow a fifteenth seal, the other fourteen, with thirteen porpoises, being found in its stomach!
According to Scammon three or four of them do not hesitate to grapple with the largest baleen whale; and, as described by Dr. Murie, "the latter often, paralysed through fear, lie helpless and at their mercy. The killers, like a pack of hounds, cluster about the animal's head, breach over it, seize it by the lips, and haul the bleeding monster underwater; and, should the victim open its mouth, they eat its tongue." In one instance he relates that a Californian grey whale and the young one were assaulted; the Orcas killed the latter, and sprang on the mother, tearing away large pieces of flesh, which they greedily devoured.
"These brutes have been known to attack a white-painted herring boat, mistaking it for a beluga; and it is stated that occasionally they will boldly lay siege to whales killed by the whalers, almost dragging them perforce under water. Near some of the Pacific sealing grounds they continually swim about, and swoop off the unwary young; even the large male sea-lions hastily retreat ashore and give these monsters a wide berth. The walrus also, with his powerful tusks, cannot keep the killers at bay, especially if young morses are in the herd. The cubs on such occasions will mount upon the mother's back for refuge, clinging for dear life, but the Orca, diving, comes suddenly up with a spiteful thud, and the cub, losing its balance, falls into the water, when in an instant it is seized by the remorseless whales." The speed of the killer whale is immense, as may be supposed when it can overtake the swift dolphins, which it catches and swallows alive. It has also been seen chasing salmon up the mouths of rivers.
The genus Orcella seems to come in between the sea and river dolphins, although Orcella fluminalis of Dr. Anderson is a purely fluviatile animal, which apparently never goes out to sea.
[NO. 258. ORCELLA BREVIROSTRIS.]
The Short-nosed Round-headed River Dolphin.
HABITAT.—The estuaries of the Ganges and Brahmaputra rivers.
DESCRIPTION.—"The head is convex from the blow-hole to the upper lip, but its sides immediately below the angle of the mouth are somewhat anteriorly convergent, but rounded; the gape posteriorly has a long upward curve; the eye, which is well developed, is near the angle at the gape, and in the adult is placed about one inch above it, with a slightly downward slope; the ear is nearly on the same level as the angle of the mouth, but is extremely small, crescentic, and not measuring more than 0·12 inch in diameter. The posterior margin of the blow-hole is immediately behind the anterior angle of the eye; the blow-hole is crescentic and unsymmetrical, being more to the left than to the right side; there are two slight eminences about one inch behind the blow-hole; the construction of the neck occurs below the ear and slightly behind it" (Anderson's 'Anatomical and Zoological Researches,' p. 370). The other characteristics are triangular flippers half as broad as long. The back fin rises behind the centre of the back; it is comparatively small, falcate, curved over the top to a blunt point, and concave behind. The line of the back is sharp from this fin down to the tail. The ventral line is the same for some inches behind the anus. The colour is dark slaty-blue above, almost black, a little paler below, without any streaks or marks, such as in O. fluminalis and Risso's grampus.
SIZE.—From snout to caudal notch, about 7 feet.