XU FOSSIL. WHALES 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 /nia already spoken of. 
Pontoporia blainvillit 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 forma 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 Aogia, 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 Kocene. 
The genus Jniopsis of Mr. Lydekker,' with one species, J. 
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 Znia 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. 
Hurhinodelphis 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 
1 Proc. Zool. Soc. 1892, p. 558. 
