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,[1] 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
- ↑ Proc. Zool. Soc. 1892, p. 558.
