RODENTIA. 237 



are spread from one end of the continent to the other, although many of them are quite limited 

 in their distribution. But one species is mentioned as occurring in South America. Of the 

 second genus, Lagomys, there is only one species known in North America, viz : L. prindeps, 

 or little-chief hare of the Eocky Mountains ; none occur in South America. 



I have thus sketched out the principal facts in regard to the distribution of the North Ameri 

 can Rodentia, leaving the details for consideration under more particular heads. It is by no 

 means unlikely that further investigation may require considerable modification in these views, 

 in the addition of many now unknown genera and species, or in the discovery of new localities. 



There are a few points in the structure of the Rodenlia to which brief allusion may be made 

 before proceeding to the consideration of the families composing the group. As already stated, 

 one family, the Leporidae, have four incisors in the upper jaw instead of two ; a second much 

 smaller pair occurring immediately behind the first, and receiving upon their surface the end 

 of the lower incisors. In the young hares there are six incisors at birth, or a little after, 

 arranged in three pairs, one behind the other. The central pair soon becomes absorbed by 

 pressure from that behind it and disappears. 



In Rodentia, generally, the incisors of both jaws are usually very long, and describe segments 

 of circles, which in the upper jaw are of smaller radius, but larger arc, than those in the lower. 

 Different groups vary considerably in the point or region attained by the root of the incisors, 

 which thus affords distinctive characters of much importance. The incisors are provided at 

 their roots with a persistent pulp which, constantly forming new tooth substance, keeps them 

 of the same length in supplying the wear of the incisive surfaces. The front of the incisors 

 being composed of enamel, and the main body of it behind the enamel of dentine, the latter 

 wears away more rapidly than the former, and thus is maintained the sharp, chisel-like edge, 

 so necessary to the gnawing function. When one incisor becomes broken off or diseased, the 

 corresponding one continues to grow at quite a rapid rate, and sometimes attains to a consider 

 able length, and may even penetrate the bone or muscle in recurving. I have seen this fre 

 quently in the case of the wood-chuck, Arctomys monax, L., as well as in other species. 



The molar teeth of the Rodentia vary in number from ^ in the hares, to | | in the rats ; 

 sometimes, indeed, one jaw may exhibit but two teeth. In some forms they are provided, like 

 the incisors, with a persistent pulp which keeps them growing all the time, and supplies the 

 wear and tear of mastication ; in others they are rooted, with distinct fangs, and do not continue 

 to grow after these are fully formed. It frequently happens that, in extreme age, the first 

 mentioned kind of molars which, in their normal state, are of uniform size, prismatic, or rounded 

 from top to base, with a diminution in the supply of pulp, throw out short irregular fangs, 

 approximating them in appearance to the rooted molars. The difference in symmetry and 

 regularity, is, however, readily noticeable. 



The molars of rodents, in many cases, are very complicated in their structure, and always 

 consist of three portions ; the hard enamel which in ridges or layers enfolds or covers the 

 softer and principal component, the dentine ; the cement, or crusla petrosa, is exterior to the 

 enamel sometimes forming a thin coat, perhaps only discernible near the base of the crown, 

 (as in many rooted teeth,) and at others filling up all its outer angles and concavities. This is 

 the case o-enerally in the molars without roots, where, occasionally, the cement may completely 

 encircle an island of enamel. 



The relative distribution of these constituents of the molars, varying so much in pattern and 

 outline, furnishes excellent subordinate determining characters, being remarkably constant in 



