230 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
the incisor-root up the ramus results in, and is betrayed by, a prominent super- 
ficial ridge or thickening that runs from the inside of the back of the condylar 
ramus downward, forward, and a little inward, to connect with the posterior 
extremity of the alveolar ridge. In Myodes, Cuniculus, and Synaptomys, this 
ridge is wanting; the whole condylar ramus being thin and flat, its inner face 
nearly plane, and separated from the posterior end of the alveolus by a strong 
sulcus. Moreover, the passage of the incisor-root so closely along the under 
border of the body of the jaw confers upon the latter a massiveness not seen 
in other genera. The protruding ends of the incisors offer nothing peculiar. 
There is not much else to note in the lower jaw. The apex of the coro- 
noid reaches nearly or quite to the level of the condyle, as in Arvicola gener- 
aly. The descending hamular process, as in Myodes, is large, strong, curves 
strongly outward, and has a much inflected lower border, so that its inferior 
aspect is a flat surface instead of a border; and the notch between it and the 
back of the condylar ramus is wide and deep. The muscular impressions 
upon the side of the jaw are deep and strong, bounded by prominent sharp 
ridges. 
As is well known, the molar teeth of Myodes (as restricted to exclude 
Cuniculus) are both individually and collectively distinguishable on sight from 
those of other Arvicoline; for, although constructed essentially upon the same 
plan of aggregated rootless prisms, their details are quite different. In ordinary 
Arvicoline, the molars as a series are sharply serrate, both internally and exter- 
nally, by reason of the acute salient and reéntrant angles they present from first 
to last, both above and below. Now in Myodes, of the upper molars the outer 
saliencies are similarly sharp, but the outer reéntrances are so deep that they 
reach almost across the teeth to the inner side; while the inner saliencies are 
so obtuse and the inner reéntrances so shallow that the inner border of the 
series as a whole is crenate instead of serrate; and in Myodes, again, of the 
under molars, the inner saliencies are sharp and the inner reéntrances deep, 
while the outer saliencies are so obtuse and the outer reéntrances so shallow 
that the outer border of the series as a whole is crenate instead of serrate. 
Thus, in Myodes, one border of both upper and under molar series is crenate, 
the other serrate, the crenation being external in the under molars, and 
internal in the upper molars. Now this molar pattern is repeated in Synap- 
fomys with such fidelity that we can indicate no difference of generic import; 
and thus the great difference between the molars of Synaptomys, and of 
Arvicola with its subdivisions, as well as of Evo‘omys, becomes evident. 
