Volume 10 — CHAPTER XII (continued 2)

There is but one species, Inia geoffrensis, which inhabits the Amazons, and grows to a length of 8 feet. Its colour variations are rather extraordinary, unless they can be set down to sex, which has been denied. Some individuals are wholly pink; others are black above and pink beneath. This Whale is believed by the Indians to attack a man in the water, and it is added that the Sotalia of the same streams will defend him from these attacks! Naturally, therefore, superstitious reverence attaches to this Dolphin, which is tiresome to the naturalist who wants specimens, as Professor Louis Agassiz found.

In the genus Pontoporia^[253] the dorsal fin is well developed and falcate. The teeth are very numerous, 200 in all. The ribs articulate as in Dolphins. The skull closely resembles that of Inia, and the scapula is, as in that genus, "normal."

The proper name for Pontoporia is really Stenodelphis, which name was first used by Gervais a month or two before Gray, who separated it from the vague Delphinus of its original discoverer, Gervais himself. It has a longer snout than Inia, which, being bent towards the extremity in a downward direction, curiously suggests the skull of a Curlew. In details, however, the skull is exceedingly like that of Inia. It is nearly symmetrical. The vertebral formula appears to be the following:—C 7, D 10, L 5, Ca 20 = 42, just one over the number of the vertebrae in Inia. The sternum is in two pieces. Of the ten pairs of ribs the first three are double-headed. These and the next have sternal moieties joining the sternum, of which the first three are ossified, the last being apparently merely a ligament.

There is a single species of the genus, P. blainvillii. This Whale is described by Mr. Lydekker as being of a clear brown colour, harmonising with the waters of the estuary of the Amazons and the La Plata which it inhabits. The same colour characterises Sotalia pallida of those parts of the world, and may be a colour adaptation. But the extant accounts of the colour of this Dolphin vary—quite possibly in accordance with real variations, such as are exhibited by Inia already spoken of. Pontoporia blainvillii is a smallish Dolphin some 4 feet in length.

Fossil Odontocetes.—Several of the existing genera of Dolphins are also known in a fossil condition, as well as Ziphioid Whales closely related to existing forms. We shall deal here only with a few genera of fossil Odontocetes which depart in their structure from existing forms.

The genus Physodon is Miocene, and has been found in Patagonia. It appears to be most nearly allied to the Physeteridae, but should probably form a distinct family. Physodon was not so large as Physeter, the skull measuring only some 10 feet. It thus comes nearer in point of size to Kogia, and it is interesting to note that its relatively-shorter snout is also suggestive of the dwarf Cachalot. The general outline of the skull is, however, more like that of Physeter, and there is the same deep cavity for the lodgment of spermaceti. The main feature of interest in the skull is the presence of teeth in both jaws, and the fact that two or three are lodged in the premaxillae. This is precisely what is found in the most ancient Whales, the Zeuglodonts.

Extinct Dolphins, apparently referable to the Platanistidae, are the most numerous among the earlier forms of Cetaceans, and it is significant that the earliest known forms of these go back to the Eocene.

The genus Iniopsis of Mr. Lydekker,^[254] with one species, I. caucasica, comes from rocks which seem to be of that age. The back part of the skull of this animal, the only part of the skull known, has the same squarish excavation of the maxillaries that characterises Inia and Pontoporia. Its lower jaw was slender and possessed numerous teeth.

The long snout and jaws of Platanistids, especially exaggerated in Pontoporia among living forms, are constantly found in these Tertiary Platanistids.

Eurhinodelphis had a beak three and a half times the length of the cranium, whereas in Pontoporia the proportions are as 2:1. The teeth too were very numerous.

The genus Argyrocetus, from Patagonian Tertiary strata, was an animal about as large as the existing Dolphin. It had the slender rostrum and numerous teeth of the Platanistids and the squared excavations of the maxillaries. Argyrocetus patagonicus possessed also archaic characters, suggesting earlier affinities still. The two condyles of the skull instead of being closely adpressed to the skull stood out in a way more like that met with in terrestrial mammals. The nasal bones instead of being abbreviated rudiments are well developed as in the archaic Zeuglodonts. The cervical vertebrae of this Whale are all perfectly free from each other and individually long. The skull is on the whole bilaterally symmetrical; this again is a feature more pronounced among the Platanistidae than among other Odontocetes. Accompanying these generalised Cetacean characters are some which show that the animal was too specialised to be the direct ancestor of any existing forms. The end of the mandible was upturned and without teeth, its form being quite unique among Cetacea. Other allied forms, such as Zarrhachis and Priscodelphinus, showed the same length of the cervical vertebrae.

A very distinct family of extinct Whales is that of the Squalodontidae. They to some extent bridge over the gap between the existing Odontoceti and the Eocene Archaeoceti (Zeuglodonts).

The skull of these Whales was on the whole Dolphin-like. But they possessed teeth which were distinctly specialised into incisors, canines, and molars. The molars have a coarsely-serrated cutting edge as in the Zeuglodonts, and are also to some extent two-rooted. But they are more numerous, and so far approximate to the conditions which characterise the more typical modern Odontocetes. Squalodon was a long-beaked form, and Prosqualodon had a skull whose proportions are nearer those of Kogia.

Sub-Order 3. ARCHAEOCETI.

This division of the Whale tribe embraces but a single family, Zeuglodontidae, of which but a single genus, Zeuglodon, can with certainty be discriminated.

Zeuglodon is an Eocene form of large size, with teeth which are limited in number and disposed in three series as incisors, canines, and molars. The molars are double-rooted, a fact which has given to the genus its name. The nasal bones being long instead of rudimentary like those of other Whales, the blow-hole lies more in the middle of the face. The skull, too, is not Whale-like in a number of other points. Thus the premaxillaries take their fair share in the outline of the upper jaw; and, furthermore, bear the incisor teeth. The parietals meet above in a crest and are not excluded from the roof of the skull. The vertebrae of the neck are in no way shortened; neither are they fused together. The ribs are double-headed, and the sternum is made up of several pieces. Some naturalists, particularly Professor D'Arcy Thompson,^[255] have assigned a relationship to the Seals to these ancient Cetacea; but others^[256] have disputed this view chiefly on the grounds that the characters which appear to be Seal-like are simply characters which are generalised and so far at most not Whale-like. Thus the long neck and the serrated character of the teeth may be accepted as Seal-like on the one hand; but on the other, a simple serrated tooth and a long neck are not by any means features of organisation which we should consider out of the way in an ancient form of Cetacean which probably preyed upon fish. The humerus of Zeuglodon, according to Mr. Lydekker, puts out of court any possible near relationship to the Seals. But the matter under dispute can be further studied by reference to the three memoirs quoted below.