MAMMALIA. 45 



ARVICOLA, Lacep. CAMPAGNOLS. Water and Field Rats. 



Three molars throughout, but without roots, and formed 

 each of triangular prisms placed on two alternate lines. Tail 

 moderately long and soft ; e.ars pretty large. 



DIPUS, Gm. Jerboas. 



Posterior extremities of a disproportionate length ; the me- 

 tatarsus of the three middle toes formed of a single bone, re- 

 sembling what is called the tarsus in Birds. Fore feet very 

 short in proportion with those behind, and furnished with five 

 toes. Sometimes to the hind feet there are two small lateral 

 toes besides the three large middle ones. Tail long and 

 tufted ; progresses by huge leaps, sometimes of seven or eight 

 feet. 



GENUS III. CASTOR, Lin. Beavers. 



Tail flattened horizontally, almost oval, and covered with 

 scales. Five toes to each foot; those behind united by a 

 membrane ; that which comes next to the thumb has a double 

 and oblique nail. Molars to the number of four everywhere, 

 and with flat crowns. The known species inhabits princi- 

 pally the solitudes of North America, and constructs for 

 the winter, upon the banks of rivers and lakes, ingenious 

 dwellings, whose regular distribution, admired by all travel- 

 lers, has been described in the most eloquent works. They 

 live on the bark of trees, and unite to the number of three 

 hundred, or thereabouts, to form colonies, and labour in es- 

 tablishing dikes against the streams. 



GENUS IV. HISTRIX, Lin. Porcupines. 



Head as if swollen ; body armed with spines, very long and 

 ringed in a regular manner with black and white ; clavicles 

 rudimentary ; molars to the number of four throughout, with 

 flat crowns, differently modified by plates of enamel, between 

 which are depressed intervals. Tongue bristled, with spiny 

 scales. Four toes before, five behind. They walk on the 

 sole of the foot, 



GENUS V. LEPUS, Lin. Hares. 



Clavicles rudimentary ; posterior limbs long; two small in- 

 cisors behind the upper ones ; five molars throughout, formed 

 each of two vertical plates soldered together. Five toes be- 



