- 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. princeps, 
or little-chief hare of the Rocky Mountains ; none occur in South America. 
I have thus sketched out the principal facts in regard to the distribution of the North Ameri- 
can Jodentia, 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 Rodentia 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 fodentia, 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 © 08 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 crusta petyosa, 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 generally 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 
