The Tetrapoda: Mammals 637 



in buildings, carrying human diseases, chewing merchandise, and kill- 

 ing poultry. The house mouse, Miis musculiis, though less destructive 

 because of its small size, is likewise a household pest. 



Throughout the United States, there are many other numerous 

 and well-known rodents. Some of these include the squirrels which 

 are important as game animals, the chipmunks, marmots, flying squirrels, 

 porcupines, lemmings, prairie dogs, and pocket gophers. All of these 

 have interesting habits, and many are of economic importance. 



Only a few rodents are important as fur bearers. Chief of these 

 are the muskrats, beavers, and chinchillas. The beavers have at- 

 tracted attention because of their ability to build dams and to make 

 ponds for their living quarters. 



Order Cetacea. — The most highly modified of all the mammals 

 are the members of this order, the whales, dolphins, and porpoises. 

 This extreme modification is associated with their marine life; all 

 have acquired a fishlike form. They have streamlined bodies without 

 a neck constriction, a fishlike tail which is horizontal rather than verti- 

 cal, and the front limbs are paddlelike while the hind limbs are lacking. 

 The skin is hairless and there is a layer of heavy insulating fat directly 

 beneath it ; the eyes are usually small, and there is no external ear. 

 The nostrils open through single or paired "blowholes" which are lo- 

 cated far back on the head. Among the larger whales, the expulsion 

 of air causes a column of water to rise. 



There is great variation in the size of the whales. The largest 

 is the bluewhale or sulphur-bottomed whale (Sibbaldus muscidus) which 

 reaches a length of over 100 feet and weight of over 140 tons. This 

 largest of all animals is a member of the whalebone or toothless group. 

 It has a series of flattened sheets of whalebone or baleen hanging from 

 the upper jaw. Water is taken in through the widely opened mouth, 

 and small organisms are sifted out by means of the whalebone and 

 swallowed. Many of these whalebone whales are of economic impor- 

 tance as a source of oil and fat. 



Of the toothed whales, one of the more highly modified forms is 

 the sperm whale, Physeter. This form has the nose enormously en- 

 larged as a storage organ for the waxy spermaceti oil. The nose is 

 without a smelling function. The sperm whales are able to dive to 

 tremendous depths, probably in pursuit of their chief food, squids 

 and octopuses. 



