1208 THE CETACEANS 



jaw and larger than the second pair. They are of moderate size, flattened from 

 side to side, pointed at the tips, and inclined directly forward. The skull lacks 

 the high crests above the opening of the nostrils characteristic of the three pre- 

 ceding genera; and the long and narrow beak is less solidly ossified than in the 

 beaked whales. 



ANCESTRAL SPERM WHALES 

 Family PHTSODONTID^ 



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 

 from Patagonia has shown that teeth were present in both jaws. These whales 



SKULI, OF THE PHYSODON. 



seem, therefore, to represent a distinct family group, from which the existing forms 

 have probably taken origin. 



THE FRESH-WATER DOLPHINS 



Family PLATANISTID^E 



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 characterized by the ribs articulating 



