150 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
various sections of the genus that it is quite worth while to recognize. The 
names Psammomys LeC. (nec Riipp.), Pitymys McMurt., and Pinemys Less. 
are mere synonyms, all having been based on the same animal (pinetorum), 
and two of them being simply amends for LeConte’s mistake in taking 
Psammomys Riipp. for his Arvicola pinetorum. ‘ Microtus” Selys. appears 
to be about the same thing as Pitymys. Hemiotomys Selys. (nec Baird) goes 
to the European amphibius. Myonomes Raf. and Pedomys and Chilotus Baird 
are tenable subgeneric names for particular American groups. Doubtless 
there are some other generic or subgeneric names that have not come to our 
notice. 
In proceeding now to define Arvicola, we may premise that our diag- 
nosis of the genus will simply be equivalent to such restriction of the char- 
acters of the subfamily Arvicoline as will exclude the Lemmings, the 
remarkable Synaptomys of Baird, and the rooted-molar group (Evotomys nob. 
= Hypudeus Keys. Blas. Bd. nec Il.). Some of the more boreal Arvicole do 
indeed closely approximate to the Lemmings in the shortness of their ears, tail, 
and feet, and in the mollipilose pelage; but the radical differences in dentition 
are never, so far as we know, obscured. It may be that there are some 
species of Arvicoline that require generic separation from Arvicola besides 
those just mentioned, but none such have come to our knowledge. 
Some of the characters we are about to give are rather those of the sub- 
family than of the genus; but the particular combination, as expressed in the 
whole paragraph, is generally diagnostic. 
Gen. Cuars.—Crowns of the (3-3, rootless, perennial, prismatic) molars 
plane, divided into several closed islands of dentine by folds of the surround- 
ing sheet of enamel that meet from opposite sides and fuse along the median 
line (cf. Evotomys); the saliencies and reéntrances of the alternating prisms 
strong and sharp, equally so on both inner and outer sides of the molar series, 
the profile of which is therefore equally serrate on both sides (cf. ALyodes, 
Synaptomys). Anterior upper molar of 5 prisms,—1 anterior, 2 interior, 2 
exterior. Middle upper molar of 4 (or 5) prisms,—1 anterior, 1 interior, 2 
exterior (the last sometimes giving off a supplementary postero-interior one). 
Posterior upper molar of 4 to 7 prisms, of which the first is always anterior 
and transverse, the last a variable treflle (C, G, WW, Y, &., in shape, 
according to subgenus or species), and the intermediate ones lateral and alter- 
nating. All upper molars subequal in length and breadth (ef. Myodes, Synap- 
