664 THE AMERICAN NATURALIST. [Vol. XXXVII. 



view it may be added that the walrus, which is a bottom-feeding 

 form living chiefly upon bivalve molluscs, has the skeleton 

 noticeably heavier than that of any other of the Pinnipedia which 

 are generally piscivorous in habit. 



The kidneys of most aquatic mammals are lobulated, Hippo- 

 potamus, Pinnipedia, Cetacea, but how this is to be explained by 

 aquatic life is not clear. 



The testes are retained within the abdomen in the Cetacea, 

 Sirenia and the true seals, Phocidas. In the less aquatic hair 

 seals, Otarikkc, they are scrotal as in the majority of mammals. 



Retia mirabilia, anastomoses of smaller arteries and veins, are 

 abundant'}' developed in the Sirenia and Cetacea. These cause 

 a slowing down of the blood stream and it has been suggested 

 that this is connected with the oxidation of the blood in these 

 forms that breathe infrequently. 



In the foregoing enumeration of adaptations the writer has 

 attempted to include only those that seem to be a result of 

 aquatic life, but in certain instances these may be open to 



the loss of hair in the Cetacea and Sirenia as due to aquatic life, 



