MURIDA—ARVICOLINA—EVOTOMYS RUTILUS GAPPERI. 147 
As usual in Arvicoling, the upper incisors are broader than they are 
deep, ungrooved, and yellow on their faces. As in all Arvicoline, except 
Myodes and Synaptomys, the under incisors run past the last molar, and reach 
up the ramus half-way to the condyle itself. 
The molar series are not quite parallel, diverging a little from before 
backward. Spite of their rootedness, in which they resemble the teeth 
of Murine, they are essentially constructed upon the plan of Arvicoline, 
being truly prismatic, with acute salient and reéntrant angles, and flat crowns 
composed of triangles of dentine surrounded by enamel walls—these walls 
meeting in several instances along the middle line of the tooth, and isolating 
dentine islands, in other instances allowing contiguous dentine islands to 
become continuous. Details of the molar crowns are as follow:— 
The front upper molar presents little or nothing characteristic, having the 
form constantly preserved throughout the subfamily. There is an anterior 
closed triangle or semilune, then an interior closed triangle, then an exterior 
one, then an interior one, then a postero-external one—five in all. The 
middle upper molar is the same essentially, but with only four alternating tri- 
angles, of which the first after the anterior one is external, the second inter- 
nal, the last postero-external. This is much as in the Pedomys and Pitymys 
section of Arvicola, and not as in the riparius section, where the last triangle 
develops a snag or spur from its inner face, making five in all, two of them 
internal. The back upper molar is the most peculiar and characteristic of 
all; indeed, it seems to be the case throughout Arvicoline that this tooth is 
diagnostic of the genera and subgenera; the sculpture of its crown certainly 
differs more than that of any other tooth. In the present case, the tooth is 
remarkable, first, for being absolutely donger antero-posteriorly than either of 
the other upper ones, which is not the case in Arvicola. In general, it comes 
nearest to the riparius type of Arvicola, having really the posterior crescentic 
loop and two distinct external triangles, so characteristic of riparius (instead 
of a simple posterior trefoil and one exterior triangle, as in Pedomys and 
Pitymys and Chilotus); but the details are even more complicated than in 
riparius. We have, first, the anterior loop quite across the tooth in front, 
then comes the first exterior triangle, then the first interior triangle; then 
all the rest is the plication of the immense posterior crescent, thus: the 
crescent has its back, which is long and nearly straight, to the outside of the 
tooth, but it throws off a spur anteriorly, forming the second exterior triangle, 
