36 CETACEANS. 
ANCESTRAL SPERM WHALES. 
Family PHYSODONTIDZ. 
Teeth of a whale from the Pliocene deposits of Europe, described under the 
name of Physodon, have been long known; but it is only recently that a specimen 

SKULL OF THE PHYSODON. 
from Patagonia has shown that teeth were present in both jaws. These whales 
seem, therefore, to represent a distinct family group, from which the existing forms 
have probably taken origin. 
THE FRESH-WATER DOLPHINS. 
Family PLATANISTIDA. 
Three species of Cetaceans, two of which are entirely confined to fresh-water, 
while the third is estuarine, differ so markedly from all other living members of the 
order as to constitute a family by themselves. They are all of relatively small size, 
and agree with the true dolphins in having a numerous series of small teeth in 
both jaws. They differ from them in that the two branches of the lower jaw are 
united by more than half their length; while the head is marked off from the 
body by a slightly constricted neck. They are also characterised by the ribs 
articulating with the vertebre in a manner very similar to that we have seen in 
ordinary mammals; and the whole of the vertebra of the neck remain separate 
from one another. 
In some respects these Cetaceans are less specialised than the other living 
representatives of the order, and are, therefore, probably more nearly allied to the 
ancestral stock. This leads Sir W. H. Flower to suggest that Cetaceans originated 
in fresh-water from land animals; but Mr. Blanford considers it more probable that 
