154 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
bicolor. Feet moderate; the fore about three-fifths as long as the hinder ; 
fore claws not larger than hind ones. 
This subgenus is fully illustrated in our article on its type, Myonomes 
riparius, where will be found an extended account of the dentition. Above 
we only give the diagnostic characters and their more prominent. coliat- 
erals. The section is especially characteristic of North America, not being, 
perhaps, exactly matched in Europe or Asia; it embraces the greatest 
number of American species of the genus, among them the most widely- 
distributed one of all, and includes, likewise, the largest of all; while none 
of them are so small as our species of Chi/otus, Pitymys, or Evotomys. 
It here becomes necessary to explain the synonymy above adduced, and 
show cause for adopting the name Myonomes for the subgenus. 
In the first and only subdivision of the American species of Arvicola 
which has, to my knowledge, been.attempted, and which was so satisfactorily 
accomplished in 1857 by Professor Baird, the term Heméotomys was adopted 
for this section. Professor Baird is, however, careful to add that this name was 
constructed by Selys-Longchamps to accommodate the European amphibius, 
terrestris, &c., the skull of which is more like Pedomys austerus ; and this is 
exactly so. Now, the very first and most essential character of Arvicola 
riparius, as compared with Chilotus oregoni, Pedomys austerus, and Pitymys 
pinetorum, is the remarkable posterior crescentic or C- or G-shaped loop on 
the back upper molar, which is unique among American species of Arvicola 
proper, though again found in another genus (Evotomys). This particular 
feature is not seen in Arvicola amphibius, where the posterior upper molar 
has but one external and one internal lateral closed triangle, and then a pos- 
terior trefoil almost exactly as in Pedomys austerus. The inapplicability, 
therefore, of the name Hemiotomys to this section is evident; for to apply it 
here, is to ignore, by implication at any rate, the chief character of the section. 
If it is to be applied to any American section, it certainly ought to be given 
to that one of which A. austerus is typical, and thus be made to supplant 
Pedomys. ‘ 
After coming to this conclusion, our first care, of course, was to see what 
European or Asiatic species our A. riparius did agree with in this particular 
pattern of the last upper molar. Among the limited number of skulls at our 
command, in addition to A. amphibius, we find as follows:—A Lapland skuil, 
labeled ‘“ratticeps” (No. 1056), has on the back upper molar an anterior loop, 
