148 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
which may be quite closed, or its area confluent with the general island of 
the crescent; behind, the crescent loops broadly, forming the end of the tooth, 
then throws out a spur on the postero-internal corner of the tooth, then in 
front of this is thrown into a large loop, which makes the second interior 
triangle, not closed, however, but continuous with the general area of the 
crescent. Thus there are in all, on the back upper molar, three exterior 
saliencies and four interior saliencies. The precise details of this tooth vary 
a little with individuals, but the pattern, as just described, we have never 
found effaced or even obscure; it is, therefore, highly diagnostic. 
The molar crowns of Arvicole in general appear to have central enamel, 
or interior folds and ridges separated from the general enamel wall that 
enfolds the teeth, but this appearance is deceptive; there is but the single 
enveloping sheet of enamel around the whole tooth, which is so deeply 
indented or folded in at the reéntrant angles of the several prisms that the 
enamel sheet of opposite sides meets and fuses along a central line, often no 
wider than a single sheet of enamel, producing the appearance just mentioned. 
Now, in Erotomys, the enamel of opposite sides, in the upper jaw, meets at 
~ various places, but the fusion is not complete; either the two sheets are 
apparent where they touch each other, or else the imperfect fusion results in 
a wall the composition of which is evident by its being broader or thicker 
than a single sheet of enamel is anywhere. And in the under series, which 
we now come to examine, the enamel walls are still more distinct, revealing 
their true relations; they never quite fuse, and, even where they press upon 
each other most closely, we can discern two distinct folds, and thus trace the 
single enveloping sheet of enamel, in and out, in its various plications, all 
around the tooth. 
The posterior lower molar affords nothing diagnostic, being, as in 
Arvicola, composed simply of an anterior, a middle, and a posterior spheri- 
cal triangle, each one of these reaching quite across the tooth, and thus lying 
directly one after the other; but a singular thing is, that the middle lower 
molar copies the same pattern. In our Arvicole proper, this middle lower 
molar has an anterior triangle, succeeded by alternating lateral triangles; but 
in this genus the lateral triangles are opposite instead of alternate, which fact, 
together with the lack of a median lengthwise line of enamel, throws the 
two lateral ones into one that reaches quite across the tooth. It is surprising, 
in this case, with essentially the same pattern, such a little variation as this 
