164 LLOYD'S NATURAL HISTORY. 



sists mainly of molluscs and crustaceans, grubbed up from 

 the mud by the aid of the tusks ; and for crushing the hard 

 shells of these the other teeth are most admirably adapted. 

 Whether, as alleged, the tusks are also employed in aiding the 

 creature to climb up on the ice, is a point in regard to which 

 there is a difference of opinion among observers. The young, 

 usually one, but occasionally two in number, are born on the 

 ice from April to June, and are tended and defended by the 

 female with remarkable solicitude and bravery. 



THE RODENTS, OR GNAWING MAMMALS. 

 ORDER RODENTIA. 



THE possession of a single pair of chisel-like incisor teeth in 

 the lower jaw, which grow continuously throughout life, and are 

 opposed by a similar pair of upper teeth, is of itself a sufficient 

 character to distinguish the Rodents, or Gnawing Mammals, 

 from all the other British representatives of the Class to which 

 they belong. In the lower jaw only this single pair of incisors 

 is developed, and the same is the case in the great majority of 

 the members of the Order as regards the upper jaw, although 

 in the Hares and Rabbits, as well as in their foreign cousins the 

 Pikas, a small and functionless second pair of upper incisors is 

 to be found behind the large ones. These chisel-like incisor 

 teeth have, as an almost invariable rule, a coating of hard 

 enamel only on their front surfaces, the result of which is that 

 they have a cutting-edge formed and kept continually sharp by 

 the action of gnawing food and other hard substances. Since 

 these teeth form a segment of a circle and grow continuously 

 throughout the life of their owner, if one by any chance happens 

 to get broken it results that the opposing tooth, having nothing 

 to wear against it, grows to a great length beyond the gums, 

 curving round, and sometimes actually piercing the skull of its 



