RODENTIA. 12.3 



under their action. To meet the wear of the enamel and other 

 parts, the teeth constantly grow in a ratio corresponding with the 

 decrease or wear. Should one tooth be lost by accident, or dis- 

 placed, the counter one of the opposite jaw becomes enormously 

 long, so as to impede its feeding, as is seen in rabbits. The mo- 

 lar teeth have flat surfaces, with ridges of enamel running trans- 

 versely across, so as to be opposed to the horizontal movement of 

 the jaw, and thus more readily grind their food. The entire 

 dental arrangement evinces admirable beauty and simplicity of 

 design. 



The Rodentia, according to De Kay, include not far from 300 

 species, spread over the globe, (except Australia,) of which 

 seventy are found in North America. They are generally inof- 

 fensive, being of a gentle and timid disposition, and trusting for 

 protection to flight or concealment ; seldom more than of a mod- 

 erate size, while a portion of them are the smallest of the mam- 

 mals. Of these last the Harvest Mouse is an example ; the lar- 

 gest Rodents are .the Beaver, Capybara, and Porcupine. The 

 Rodents feed upon the harder sort of vegetable matter, as 

 nuts, grain, roots, twigs, etc., (except rats and mice, which are 

 omnivorous, eating anything that comes in their way, as most 

 house-keepers know to their sorrow.) The Rodents have gener- 

 ally six or eight young at a birth, and this two, three, and even 

 four times in a year. They are, however, kept from overrun- 

 ning the earth by the rapacity of beasts and birds that live upon 

 them. 



Many are remarkable for their soft and beautiful fur. The 

 Beaver, Chinchilla and Grey Squirrel are valuable in commerce. 

 Some of them, as the squirrel and dormouse, use the fore paws 

 to convey food to the mouth, to hold an object, and to climb. 

 The form of the body is usually more or less conical, the chest 

 and shoulders being small, whilst the loins and haunches are 

 robust and muscular ; the hinder limbs are longer than the fore 

 ones, whence their movement is that of leaping or hopping 

 along. "Most of them are nocturnal or crepuscular in their 

 habits; many dwell in burrows ; some conceal themselves amidst 

 herbage, some among the foliage of trees, and some build for 

 themselves habitations which have exteited the interest and admi- 

 ration of men." (Pict. Museum.) 



We arrange the numerous animals of this order into eight fam- 

 ilies, viz. : 1. SrrTRina:, (Squirrels,) 2. ARCTOMYDJE, (Marmots;) 

 3. GERBILLTDJE, (Jerboas;) 4. CASTORIDJE, (Beavers;) 5. HYS- 

 , (Porcupines;) 6. MURIDJE, (Rats and Mice ;) 7. CAVI- 

 (Cavies :) 8. LEPORID.E, (Hares) Our limits will not 



