Cope.] 90 April 7, 
cipally in the form of the anterior lower molar; (see accompanying cut). 
The anterior lobe of this tooth is much shortened and crescentic, the in- 
ner horn of the crescent being the apex of aridge of the tooth. Thus there 
are five internal and three external ridges to the tooth. Triangles of 
inferior series (1) 171 three lobed ; (2) 111 ; (8) 1, 1, 1. The anterior loop 
of the second molar is contracted, outlining two triangles. the lobes of 
the third are angular sub-crescentic, the anterior trapezoid. This molar 
differs distinctly in structure from that of the next species, q.v. The 
A. involuta is nearer the A. pinetorum, and is of the same size. 
ARVICOLA sIGMopUs, Cope. 
Species nova. 
This species belongs to the same group (as characterized by dentition), 
as the last two, and is of about the same size, viz: about that of our com- 
mon A. riparia. It is represented by three imperfect mandibular rami, 
two with dentition complete, the other with the posterior molar only 
wanting. Its characters are near those of A. austera, Lec., as pointed 
out by Prof. Baird. It differs from the A. didelta and A. involuta of 
the present paper, in the five lobed anterior loops of the first inferior mo- 
lar. The loop has, therefore, besides the two basal unenclosed triangles, 
a smaller projecting angle on each side, and the terminal slightly angu- 
lated lobe. In the most typical specimen, the median angles of this lobe 
are as prominent (fig. a) as the basal, or the triangles, though the loop of 
the lobe isnot angulated at the end. 
There are, thus, five internal and four external ridges of the tooth. 
The triangles are as usual 1271, 5 lobed, (2) 141, (8), 1,1, 1. The ante- 
‘rior loop of the second is contracted as in the two preceding species. 
The third is quite different from that described under the head of A. in- 
voluta, and that figured by Baird for A. pinetorum. Thus, its three loops 
are chiefly extended inwardly, their outer angles projecting very little 
beyond the point of junction ; they form a w or sigma-shaped grinding 
surface, whence the name of the species. Prof. Baird’s figure of A. aus- 
tera represents the first inferior molar of this species exactly, but is very 
different in form of the last, which is like that of the A. involuta. Should 
however,. the character as here described in this tooth of A. ségmodus, be 
found to eccur in the A. austera, the former name will become a synonyme 
of the latter. 
Measurements. 
M. 
Length grinding surface infer, molars, (NOv.1.).---4..-+.2+1.s008 0.0065 
es se foe et a be esas ft ays 0038 
«¢ fang and crown Ky ie os Bee ORG sa asco 004 
WHC MIRICILOM ICIBOD sac c sci sous ope oa Oa eno nee es Gee, 0015 
The dental series of the more typical specimen, whose m. 1 is outlined 
in fig. a, is smaller and relatively a little narrower than the others. 
The supposed superior maxillary dentition is represented by both series, 
that of the left side lacking the first molar, with the palatine surface and 
