Risso’s dolphin, Grampus griseus, represents another genus, characterized by the absence of teeth in the upper and the small number of these in the lower jaw (3 to 7 on each side, and confined to the region of the symphysis). Vertebrae: C 7, D 12, L 19, Ca 30; total 68. General external characters much as in Globicephalus, but the fore part of the head less rounded, and the flippers less elongated. G. griseus is about 13 ft. long, and remarkable for its great variability of colour. It has been found, though rarely, in the North Atlantic and Mediterranean.
The common dolphin (Delphinus delphis) is the typical representative of a large group of relatively small species, some of which are wholly marine, while others are more or less completely fluviatile. They are divided into a number of genera, such as Prodelphinus, Steno, Lagenorhynchus, Cephalorhynchus, Tursiops, &c., best distinguished from one another by the number and size of the teeth, the form and relations of the bones on the hinder part of the palate, the length of the beak and of the union of the two halves of the lower jaw, and the number of vertebrae. For the distinctive characters of these genera the reader may refer to one of the works mentioned below; and it must suffice to state that, collectively, all these dolphins are characterized by the following features. The teeth are numerous in both jaws, and more than 20⁄20 in number, occupying nearly the whole length of the rostrum, and small, close-set, conical, pointed and slightly curved. Rostrum more or less elongated, and pointed in front, usually considerably longer than the cranial portion of the skull. Vertebrae: C 7, D 12-14, L and Ca variable; total 51 to 90. Flippers of moderate size, narrow, pointed, somewhat sickle-shaped, with the first digit rudimentary, the second longest, third nearly equal, and the fourth and fifth extremely short. Externally the head shows a distinct beak or pointed snout, marked off from the antenasal fatty elevation by a V-shaped groove. Dorsal fin rather large, triangular or sickle-shaped, rarely wanting. A curiously marked brown and white species, perhaps referable to Lagenorhynchus is found on the fringe of the Antarctic ice (see report on the zoology of the “Discovery,” published in 1907 by the British Museum). See [Dolphin].
Extinct Cetacea.
At present we are totally in the dark as to the origin of the whalebone-whales, not being even assured that they are derived from the same stock as the toothed whales. It is noteworthy, however, that some of the fossil representatives of the latter have nasal bones of a type recalling those of the former. Such fossil whalebone-whales as are known occur in Pliocene, and Miocene formations are either referable to existing genera, or to more or less nearly related extinct ones, such as Plesiocetus, Herpetocetus and Cetotherium.
The toothed whales, on the other hand, are very largely represented in a fossil state, reaching as low in the geological series as the upper Cretaceous. Many of these present much more generalized characters than their modern representatives, while others indicate apparently a transition towards the still more primitive zeuglodonts, which, as will be shown later, are themselves derived from the creodont Carnivora. In the Pliocene deposits of Belgium and England are preserved the teeth and other remains of a number of cetaceans, such as Physodon, Encetus, Dinoziphius, Hoplocetus, Balaenodon and Scaldicetus, more or less nearly related to the sperm-whale, but presenting several primitive characters. A complete skull of a member of this group from the Tertiary deposits of Patagonia, at first referred to Physodon, but subsequently to Scaldicetus, has a full series of enamelled teeth in the upper jaw; and it is probable that the same was the case in other forms. This entails either a modification of the definition of the Physeteridae as given above, or the creation of a separate family for these primitive sperm-whales. In other cases, however, as in the Miocene Prophyseter and Placoziphius, the anterior portion or the whole of the upper jaw had already become toothless; and these forms are regarded as indicating the descent of the sperm-whales from the under-mentioned Squalodon. The beaked whales, again, are believed to be independently descended from the latter type, Berardius being traced into the Miocene Mioziphius, Anoplonassa and Palaeoziphius, the last of which shows signs in its dentition of approximating to the complicated tooth-structure of the squalodonts.
Another line of descent from the latter, apparently culminating in the modern Platanistidae, is represented by the family Eurhinodelphidae, typified by the European Miocene Eurhinodelphis, but also including the contemporary Patagonian Argyrocetus and the nearly allied European Cyrtodelphis. All these were very long-beaked dolphins; and in Argyrocetus, at all events, the occipital condyles, instead of being closely pressed to the skull, are as prominent as in ordinary mammals, while the nasal bones, instead of forming mere rudimentary nodules, were squared and roofed over the hind part of the nasal chamber.
In the Miocene Squalodon, representing the family Squalodontidae, the dentition is differentiated into incisors, canines and cheek-teeth, the hinder ones of the latter series having double roots and compressed crowns carrying serrations on the hinder edge; generally the dental formula has been given as i. 3⁄3 , c. 1⁄1 , p. 4⁄4 , m. 7⁄7 , the single-rooted cheek-teeth being regarded as premolars and those with double roots as molars. Dr Abel is, however, of opinion that the formula is better represented as i. 3⁄3 , c. 1⁄1 , p. 8 or 9⁄9, m. 3⁄2 ; the teeth reckoned as molars corresponding to those of the creodont Carnivora. The single-rooted cheek-teeth are regarded as due, not to the division of double-rooted ones, but to the fusion of the two roots of teeth of the latter type. In Squalodon the nasal bones were of the modern nodular type, but in the Miocene Patagonian Prosqualodon they partially covered the nasal chamber.
At present there is a gap between the most primitive squalodonts and the Eocene zeuglodonts (Zeuglodontidae), which are regarded by Messrs Max Weber, O. Abel and C.W. Andrews as the direct forerunners of the modern-toothed whales, forming the suborder Archaeoceti. It is, however, right to mention that some authorities refuse to admit the relation of the Archaeoceti to the whales.
In the typical zeuglodonts the long and flat skull has large temporal fossae, a strong sagittal crest, a long beak formed mainly by the premaxillae (in place of the maxillae, as in modern whales), and long nasal bones covering over the nasal chamber, so that the nostrils opened about half-way down the beak. All the cervical vertebrae were free. Normally the dentition in the typical genus Zeuglodon (which is common to the Eocene of North America and Egypt) is i. 3⁄3 , c. 1⁄1 , p. 4⁄4 , m. 3⁄3 ; the cheek-teeth being two-rooted, with compressed pointed crowns, of which the fore-and-aft edges are coarsely serrated. In the Egyptian Zeuglodon osiris the number of the molars is, however, reduced to 2⁄3 , while some of the earlier cheek-teeth have become single-rooted, as in the squalodonts. The probable transitional form between the latter and the zeuglodonts is the small Microzeuglodon caucasicus described by the present writer, from the Caucasus. As regards the origin of the zeuglodonts themselves, remains discovered in the Eocene formations of Egypt indicate a practically complete transition, so far at least as dental characters are concerned, from these whale-like creatures to the creodont Carnivora. In the earliest type, Protocetus, the skull is practically that of a zeuglodont, the snout being in fact more elongated than in some of the earliest representatives of the latter, although the nostrils are placed nearer the tip. The incisors are unknown, but the cheek-teeth are essentially those of a creodont, none of them having acquired the serrated edges distinctive of the typical zeuglodonts; and the hinder premolars and molars retaining the three roots of the creodonts. In the somewhat later Prozeuglodon the skull is likewise essentially of the zeuglodont type, although the nostrils have shifted a little more backwards; as regards the cheek-teeth, which have acquired serrated crowns, the premolars at any rate retain the inner buttress supported by a distinct third root, so that they are precisely intermediate between Protocetus and Zeuglodon. Yet another connecting form is Eocetus, a very large animal from nearly the same horizon as Prozeuglodon; its skull approaching that of Zeuglodon as regards the backward position of the nostrils, although the cheek-teeth are of the creodont type, having inner, or third, roots. It is noteworthy that Zeuglodon apparently occurs in the same beds as these intermediate types.
It follows from the foregoing that if zeuglodonts are the ancestors of the true Cetacea—and the probability that they are so is very great—the latter are derived from primitive Carnivora, and not, as has been suggested, from herbivorous Ungulata. The idea that the zeuglodonts were provided with a bony armour does not appear to be supported by recent discoveries.