FKESH-WATER DOLPHINS. 65 



are the most primitive of all existing cetaceans. At 

 present we have, indeed, no evidence of fossil forms allied 

 to the susu or Gangetic dolphin (Platanista) ; but in the 

 older Tertiary deposits, both of the United States and 

 Europe, there occur the remains of dolphins evidently 

 nearly allied to the two existing South American species 

 (Inia and Stenodelphis) , and thus clearly proving the 

 antiquity of those types. It may be added that it is 

 probable that the ancestors of the cetaceans which first 

 took to an aquatic life were inhabitants of fresh-water, 

 and it is therefore only what we should expect that the 

 most primitive of the existing representatives of the order 

 were likewise of fluviatile habits. On the other hand, it 

 must be confessed that the similarity in structure of the 

 widely separated existing fresh-water dolphins is some- 

 what difficult to account for on this hypothesis. 



