39S 



QUADRUPEDS. 



chisels, and are thus fitted for chipping off and destroying piece- 

 meal the hardest and most intractable materials. In order that 

 they may be bette'r capable of such employment, these chisel- 

 teeth are provided in front with a thin layer of enamel, hard as 

 the hardest steel, whilst behind, they are composed of a much 

 softer material called ivory, so that they always present a sharp 

 cutting edge. These teeth are constantly growing from the pulpy 

 core at their base ; but as those of the upper jaw meet those of 

 the lower at their tips, they are perpetually worn away by their 

 action upon each other and upon the hard food which they arc 

 formed for nibbling, so that the wearing away of the extremities 

 and the growth from the bases balance each other with exact pre- 

 cision. When, however, by accident, an opposing incisor is lost, or 



when by thedistorted union 

 of a broken jaw the lower 

 incisors no longer meet the 

 upper ones, as sometimes 

 happens to a wounded 

 hare, the incisors grow un- 

 til they project like the 

 tusk of an elephant. The 

 Rodents are all timid and 

 feeble, trusting for self- 

 protection to flight or con- 



FIG. 44 2.- SKULL OF PORCUPINK. cealment. The prey of 



ferocious beasts and birds 



and reptiles, their fertility, by a wise provision, counterbalances 

 their annual diminution. Spread over the earth from the equator 

 to the coldest latitudes, they tenant rocks and mountains, plains 

 and woods, and often devastate the cultivated domains of man. 

 About half of the Mammals known belong to this Order, which 

 contains above six hundred species. Of these, three hundred 

 and six are of the family Miirida (Rats and Mice), while the 

 Squirrels (Sciurida) contain exactly half that number. Fifteen 

 species are enumerated by Professor Bell as belonging to the 

 British Isles. The Rodentia are classed as in the table (p. 399). 



The Beavers (Castor) are distinguished from all other Rodents by their 

 tail, which is horizontally flattened, of a nearly oval form, and covered with 

 scales. They have five toes on every foot, and those of the hinder feet are 

 webbed. 



The Beaver (Castor Fiber}* is an inhabitant of the most solitary parts of North 

 * Fiber, an old name for the beaver, now used as the specific name. 



