gophers. Rabbits (Lagomorpha) are often included in this group, although 

 they differ from the rest in having two tiny teeth set directly behind the 

 two large upper incisors. The members of this group are very difficult to 

 identify, as distinguishing characters are based on skull and tooth distinc' 

 tions, differences in proportion and shades of coat coloring. The Insectivora 

 are probably the least familiar group. Here belong the moles, animals adapted 

 for underground life by short fur that lies smooth when brushed either way, 

 fore limbs widened and shortened, hands enlarged for digging, and eyes re 

 duced to the si::e of pin-heads and covered with thin skin — and the shrews, 

 which look like velvety-haired, short-tailed mice but differ from mice in denti- 

 tion. The Carnivora include the foxes, wolves, cats, bears, raccoons and 

 ring-tailed cat, and the weasels. Some of these are almost entirely carnivorous, 

 v^hile others have become adapted to a more varied diet. These habits are 

 interestingly correlated with the form of the cheek teeth. In the more strictly 

 carnivorous forms the last premolar or the first molar is of the sectorial or 

 carnassial type, with a sharp edge and flattened side, and engages a similarly 

 modified tooth in a shearing action. In more omnivorous forms, such as the 

 raccoon, these teeth have become broader and serve for crushing or grind- 

 ing rather than shearing. In still more omnivorous forms, such as the bears, 

 fond of insects and berries, the molars are definitely of the grinding type and 

 the premolars are quite small and often shed. Although outside the range 

 of this book, the seals (Pmnipedia) may be mentioned as an extreme ex- 

 ample of tooth change. Their fish-eating habits have permitted a decided 

 modification in cheek teeth, which are small and often each with three points 

 or cusps, a character usually regarded as primitive, since it is common in 

 fossil forms. In fact one order of extinct mammals has received the scientific 

 name of Triconodonta. 



The problems of the distribution of mammals and the factors governing 

 scarcity or abundance make an interesting ecological study. Most of the 

 larger mammals or those sought by man for fur, food or sport are being 

 rapidly killed off except as laws providing for regulation or restriction of hunt- 

 ing and trapping prevent total extermination. Deer, for example, would 

 long since have become extinct were it not for closed seasons and other hunt- 

 ing restrictions. At present the Virginia deer is increasing its numbers in 

 some sections of the country under regulated control of hunting. The opos- 

 sum is one of the few mammals that is extending its range in spite of apparent 

 handicaps of small brain and little protection. Every few years it is reported 

 from points farther and farther north; since its introduction into California 

 several years ago, it has increased its numbers prodigiously. In general, the 

 mammals of the greatest abundance and widest distribution are those that are 

 adaptable in food habits. Animals of omnivorous diet, such as most of the 

 rodents, are usually plentiful. The muskrat dines on succulent water plants 



465 



