CETACEANS. 



ANCESTRAL SPERM WHALES. 



Family PHYSODONTID^?. 







Teeth of a whale from the Pliocene deposits of Europe, described under the 

 name of Physodon, have been long known j 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 PLATANISTID^. 



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 vertebrae in a manner very similar to that we have seen in 

 ordinary mammals ; and the whole of the vertebras 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 



