520 U. Ss. P. R. R. EXP. AND SURVEYS—ZOOLOGY—GENERAL REPORT. 
is about one-third the distance from the incisors to the occiput. The line of molars is short, 
only about one-fifth the length of the skull, (.20.) 
A rather striking peculiarity of this species and its allies is seen in the construction of the 
bony palate, which ends sharply like a thin shelf, with a cavity below and behind it, (when the 
palatal surface is superior,) the shelf with nearly a straight edge. In the other American types 
there is a step from the palatal surface to a fossa opposite the inner face of the posterior upper 
molar, from which there is an additional step to the plane of the basi-sphenoid and basi-occipital, 
the palatal surface ending in an acute angle behind. The incisive foramina are unusually long 
and open. 
The lower jaw is lower than in the large Arvicolae, the coronoid process shorter, not rising 
to the level of the top of the condyloid, and more erect posteriorly. The descending ramus is 
longer, narrower, and more horizontal. 
The incisors are small, and narrower transversely than antero-posteriorly. The molars are 
small and narrow. There is little characteristic in the anterior upper one. The middle is 
composed of one anterior, one interior, and two exterior closed triangles, four in all; the pos- 
terior an outer one; instead of five, the posterior an inner one, as in A. riparius, &c. The 
posterior upper molar is longer than the middle one. It is composed of an anterior triangle, 
two external and one internal, and terminating in a bent crescent-like loop, the back of which 
is external and nearly straight or slightly concave. There are four salient angles on the inner 
side, and three on the outer, besides the long nearly straight line of the posterior loop. 
In the molars of the lower jaw the outlines of the enamel ridges in the crowns are formed 
entirely by the indentations of one single outer enveloping sheet of enamel. When these 
indentations or re-entrant angles meet from opposite sides in the crown, their parietes are dis- 
tinctly double, instead of fusing into one. There is thus a much less tendency to lateral enclosed 
triangles, the triangular spaces when indented going usually entirely across the tooth. The 
posterior molar, as usual, is composed of three triangles, or sub-triangular spaces. The middle 
molar has two indentations on either side, which meet and divide the crown into three triangles, 
each extending across the tooth. There are three salient angles on each side, which, like the 
indentations, do not alternate with each other on opposite sides, but are opposite. The anterior 
molar has three indentations on the outside, and four inside, with four salient angles outside, 
and five inside, counting the angular sides of the anterior loop, (which are sometimes quite 
obsolete.) The dentine of the anterior loop with its angular edges is continuous with that of 
the succeeding inner salient angle. 
In the female skull (1957) the posterior external triangle of the posterior upper molar is not 
cut off from the posterior loop, but, on account of the shallow indentation behind it, is continuous 
with the loop. The anterior outer re-entrant angle of the middle lower molar slips a little past 
the opposite one, so as to form an imperfect lateral triangle on either side, instead of a continuous 
one completely across. The dentine of the anterior loop of the anterior lower molar is con- 
tinuous with that of the first external salient angle as well as the internal ; indeed, there is a 
constant tendency to variation in the closeness of approximation of the opposing re-entrant 
angles, and the consequent number of complete enclosures. The angular outlines of the enamel 
lines are generally more rounded off. The skulls of specimens from Massachusetts in general 
agree in all essential respects, allowing for their belonging to younger animals. ‘The inter- 
orbital portion of the skull is wider, measuring .18 of an inch. The muzzle is shorter and 
more arched. There is no important difference in the teeth, 
