MURIDA—ARVICOLINA—SYNAPTOMYS. 931 
In Synaptomys, the front upper molar is composed of five prisms, and its 
crown of as many closed dentine islands surrounded by enamel walls:—an 
anterior loop across the tooth, then a small internal triangle close to the first, 
then a larger external triangle widely separated from the first by a reéntrance 
reaching the inner side, then a small internal triangle close to the last, then 
a postero-external loop, the latter separated from the last by a very deep 
reéntrance similar to the other reéntrance. The middle upper molar has four 
dentine islands:—an anterior loop reaching across the tooth, then a large 
external triangle that also extends across the tooth, then a small strictly 
internal triangle, and then a directly posterior loop. The last upper molar 
has likewise four dentine islands, all of which are rather transverse loops 
than triangles, and really reach from side to side of the tooth; the first of 
them is separated from the next, and this second from the third, by a deep 
external reéntrance, while the fourth is separated from the third by a similar 
internal reéntrance. All this of the upper molars is exactly as in Myodes. 
The front under molar, like the front upper one, has five dentine islands: — 
an anterior loop somewhat trefoil-shape or triangular reaching across the 
tooth, then a large internal lateral triangle, then a very small external lateral 
triangle, then a large triangle reaching nearly across the tooth, and finally 
a posterior loop reaching quite across. But the external lateral is so minute 
and inconsiderable, and the two internal saliencies between the anterior 
and posterior ends of the tooth make such large triangles, that the tooth 
might be thus described: an irregular anterior trefoil, a regular posterior 
crescent, with two large triangles between, one or both of which may bear 
a small spur, isolated or not, upon its exterior angle. The middle under 
molar has four triangles, of which the first one and last two are large and go 
quite across the tooth; the other one is a minute external appendage to the 
first one, hardly distinguishable. In this tooth only is there a slight difference 
from Myodes, which has an additional triangle anteriorly, making five in all. 
The back lower molar is almost exactly like the middle one, having the same 
four triangles; it is merely a little smaller, but the minute external triangle 
may not be obvious, leaving apparently three. Both the middle and lower 
one may be described as being in general shape like the letter M, with the 
two legs and middle saliency of the M internal and acute, and the two horns 
of the M obtuse and external. 
The molar series, as wholes, are of about ordinary length, measuring 
0.26 to 0.30 in length; their length, relative to the skull, is as 0.28 to 1.12, or 
