Cope.] 88 [April 7, 
idea of its form, That this is not one of the species of Pitymys, in 
i which the basal lobe of the anterior trefoil has been cut off by unusual 
\| inflexion of the enamel angle, is demonstrated by the structure of the 
second molar, which is precisely that of typical Arvicola, all the triangles 
from the posterior being isolated and alternating, producing the formula, 
120. The third molar has the usual formula, 1-1-1, the posterior two 
lobes being crescentic, the anterior trapezoid. 
| Measurements. 
| M. 
Length grinding surface inferior molars, (No. 1)........... soa W0UUS 
ie es ee eS 008 
e lapg aud Growl... . en ea eee 005 
| The structure of the molar triangles, i. e., their acuteness and thinness 
of enamel, induces me to descrite here, without any certainty of refer- 
ence, the superior molar teeth of one individual found near the same time. 
| The formulae of the superior molars are (1) 1 4, (2) 14, (3) ? this, so far 
i] as known, identical with that of A. dédelia. In another specimen repre- 
il sented by incisor and sup. m. 1., the former has an oblique antero-exter- 
| nal face, with narrow truncate outer face ; enamel not striate, emarginate ‘ 
i at the cutting edge. In A. didelta, (fossil, below), the antero-external 
face is more oblique, and without defined external plane ; the end is not 
| emarginate (in one specimen). This tooth appears to be relatively small- 
er and weaker in the ?.A. speothen. 
i M. 
| Length fang and crown, ist superior molar (No. 8)......... Vans sees 004 
| Width enamelled face incisor  ‘ ae Be eee 001 
ARVICOLA TETRADELTA, Cope. 
| Sp. nov. 
|| Represented bya portion of the cranium, which embraces the 2d and 3d 
i superior molars, and parts of the m.1, and incisor. The formula of the 
fina ert hin ic ace a tes tte le OOH 
\ 
H 
2 
two molars perfectly preserved is 14, 13%. The terminal triangles are as 
well developed inside as outside ; the others are rather small and obtusely 
angulated. Those of the m. 8, are entirely separated, and the last is not 
followed by a loop, but is completely enclosed behind by the vertical 
enamel plate, which bounds the corresponding triangle in the m. 2. This 
isa character which distinguishes this species from any other of the 
