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 {383} 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.