154 MONOGRAPHS OF NORTH AMERICAN RQDENTIA. 



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, Myonomrs 

 riparius, where will he found an extended account of the dentition. Above 

 we only give the diagnostic characters and their more prominent collat- 

 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 tliein 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 Hemiotomys 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 

 pinctorum, 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 skull, 

 labeled "rattkeps" (No. 105G), has on the back upper molar an anterior loop, 



