CLASS MAMMALS: ORDER RODENTIA. 



95 



Mg. 155. 



Skull of a Rodent. 



up hedgehog into the water ; when the frightened animal, 

 unrolling itself for an instant, will be caught by its crafty 

 enemy before it can close up again. 



ORDER RODENTIA. 



General Characteristics. The Rodents (gnawers) lack 

 canine teeth, but have two upper and lower incisors fitted to 

 gnaw bark, roots, woody stems, 

 nuts, etc. These are long, slightly 

 curved, chisel-edged, and deeply 

 rooted in the jaw. Friction upon 

 each other and constant growth at 

 the base keep the edges sharp.* 

 The molars are generally ridged 

 transversely and the lower jaw 

 moves backward and forward to 

 aid in mastication. Most of the order have clavicles, and 

 hence rotary motion of the fore paws, between which they 

 hold their food while gnawing. 



I Jaculidae, Jumping Mo use. 



I Muridae, Mouse. 



> Geomyidae, Gopher. 

 5 Castoridae, Beaver. 

 \ Sciuridae, Squirrel. 



5 Hystricidae, Porcupine. 

 Caviidae, Guinea Pig. 



> Leporidae, Hare. 



Jaculidae. Jumping Mice are remarkable for their cheek- 

 pouches and long, zigzag leaps. There is but a single species 

 in North America. 



* If one of the incisors be broken off, its opposite continues to grow, some- 

 times curving, in which case at length it locks the jaw, and the wretched animal 

 starves to death. 



