CLASS MAMMALS. 



351 



mammals, of aiding in the retaining of the heat of the 

 body, is performed in whales by a thick layer of fat, or 

 blubber, under the skin. The nostril is at the top of the 

 head for convenience in breathing or " blowing." 



FIG. 149. Young 



sum (Didelphys virginiand). Photo by Folsom. 



Questions on the Figure. Of what conceivable value to the 

 animal is the prehensile tail? In what other groups of animals is the 

 tail prehensile? What are the habits of the opossum? How is 

 the species distributed on the earth? What are its nearest relatives? 

 Where are they found? 



347. The Rodents (Rats, Mice, Rabbits, Squirrels, 

 Beavers, and Porcupines) . These are numerous in species 



