20 THE AMERICAN BEAVER. 



horizontal movement of the jaw, and better to assist 

 trituration."^ * * * * 



"Throughout the present group, the brain is almost 

 smooth, and without furrows. * * * Ju 

 a word, the inferiority of these animals is perceptible 

 in most of the details of their organization." 



Baird remarks upon the rodents: " They exist in 

 all parts of the world, and are especially abundant in 

 America, which contains nearly as many species as all 

 the rest of the world put together. South America, 

 however, counts more species than the northern half 

 of the New World, the preponderance being caused 

 principally by the large number belonging to the genus 

 Hesperomys, of which our little deer- or white-footed 

 wood-mouse, is a familiar example."^ 



Waterhouse introduces the order Rodentia in the 

 following language : " The Rodentia, so called from 

 their gnawing propensities, form one of the most 

 clearly defined groups of the mammalia; a group 

 which has representatives in all parts of the world,, 

 and the species of which are very numerous. They 

 feed upon vegetable substances, and are of small size, 

 few exceeding the common hare in bulk. The most 

 striking characters of the rodents are those furnished 

 by the teeth; the long, vacant space which separates 

 the incisors in front, here adapted for gnawing, from 

 the masticating teeth behind. * * * * 



Sometimes the width of the incisor is very great, and 

 exceeds the depth; the rodents which burrow, and 

 live almost entirely under ground, present this form 



^ Animal Kingdom. Carpenter and Westwood edition, p. lOT. 

 ^ Explorations for a Railroad Route, etc. to the Pacific, viii. 235. 



