7] 89 
[Cope. 
genus Arvicola which has been described from North America, of which 
the corresponding tooth is known. ‘The last triangle is slightly angulate - 
in posterior outline. 
M. 
Length grinding face of m. sup. 2 and 3............ ih Lagan 0.0087 
It is only necessary to compare this species with tho A. speothen and 
A. tinvoluta, in which, unfortunately, the corresponding tooth is unknown. 
Its small and obtuse triangles distinguish it from the former. As reduc- 
tion of the terminal loop of the m. inf. 1 is characteristic of the latter, 
the present tooth might be suspected to belong to it, but there is a real 
increase in the number of triangles over that of the most nearly allied 
species. A. didelta, and of the section Pitymys, to which it belongs, 
pointing most to the sections Jsodelia or Arvicola. Size, .25 less than A. 
speothen. 
ARVICOLA DIDELTA, Cope. 
Species nova. 
Represented by the mandibular rami of five, and superior dentition of 
probably three individuals. One imperfect cranium contains the denti- 
tion of both jaws, thus fixing the relations of fragmentary specimens, 
especially in the more important relation of the anterior, inferior and 
posterior superior molars. The characters of these show that it is allied 
to the A. pinetorum. The accompanying cuts illustrate the form of the 
first inferior molar tooth. The ridges are four internal and three exter- 
nal. The second molar exhibits the formula 141, the anterior area with 
an approach to division into two triangles, alternating. This peculiarity 
is not seen in Prof. Baird’s figure of A.pinetorum, 1. c. liv. 1719, nor does 
the latter represent the loops of the last molar, as exhibited by our speci- 
mens. In the figure they are oval, in our specimens angulate crescentic. 
As the figure does not agree with the description, I do not rely on it for 
these details. 
A more important difference is seen in the structure of the superior 
third molar, which Baird describes and figures as having but three isola- ’ 
ted areas, the lateral angles being sub-opposite and confluent medially. 
In A. didelta, there is an internal and an external triangle, each entirely 
isolated, besides the anterior and the posterior loops. The last differs a 
little in its developments ; in one it is broad heart-shaped, the apex pos- 
terior ; in another elongate, the sides a little concave ; in a third, more 
elongate and concave. The formula of triangles of both series then is: 
mip. te lee, abe Set. Inter aal. 25d, 852 
Prof, Baird does not describe the triangles of the posterior superior 
molars in the Arvicola austera as isolated, though it might be inferred 
from his language. His figure, however, resembles that of the A. pine- 
torum. (See Baird, U. 8. Pac. R. R. Surveys, viii, 539.) 
ARVICOLA INVOLUTA, Cope. 
Species nova. 
Established on a nearly complete ramus mandibuli, with dentition per- 
fectly preserved, It is nearly allied to the A. pinetorum, differing prin 
A. P. S—VOL. XII—L 
