444 ZOOLOGY. 



Order 6. Rodentia (gnawing animals}. The rodents are 

 small mammals with clawed digits. They have no canine teeth, 

 but have well-developed chisel-shaped incisors which continue 

 to grow as they are worn at the extremity. The chisel edge is 

 preserved by the fact that the enamel is chiefly in front, and 

 the exposed, softer dentine behind is worn away more rapidly 

 by being used. The brain is smooth. The mammae are 

 abdominal. The rodents have world-wide distribution, and are 

 especially well represented in North America. The principal 

 types are the rats and mice, many of which are close followers 

 of civilized man ; squirrels and prairie dogs, beavers, hares and 

 rabbits, and porcupines. The rodents feed on vegetable diet, 

 and are destructive of many plants and grains which man uses 

 for food. Notwithstanding man's efforts to destroy them their 

 remarkable power of reproduction enables the more aggressive 



FIG. 236. 



FIG. 236. The Jumping Rat (Perodipus richardsoni), adult male. Photographed from 

 life by Dr. R. W. Shufeldt. 



Questions on the figure. What order of mammals is illustrated by 

 this form? What explanations are offered as to the cause of the light 

 color of the belly and the dark color of the backs of animals? Of what 

 conceivable advantage is the difference in coloration ? How does the tail 

 of this species compare with that of our common rat? 



