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154 



MONOGRAPHS OF NOllTn AMERICAN RODENTIA. 



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l)ic()l(>r. Vc.ci inodiM-alc; tlio fore al)out thrcc-fiftlis as long as tlie hinder; 

 fore claws not larger tiiaii hind ones. 



This siihg(!niis is fully illnstrated in our article on its ty])e, Myonoms 

 ripnriux, where will l)e found an extended account of the dentition. Above 

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

 (•ri;ls. The section is cs|)ecially characteristic of North America, not being, 

 perhaps, exactly matched in Europe or Asia; it embraces the greatest 

 numl)er of Anun-ican species of the genus, among them the most widely- 

 distributed one of all, and includes, likewise, the largest of all; while none 

 of th(!m are so small as our species of Chilolu.s, Pitymi/s, or Evotomys. 



It here becomes necessary to explain the synonymy above adduced, and 

 show cause for adojjting the name Myonomes for the subgenus. 



In the first and only subdivision of the American species of Arvkola 

 which has, to my knowledge, been atten.,,ie(l, and which was so satisfactorily 

 accomi)lishe(l in 1B57 by Professor Baird, the term Ilemiotomys was adopted 

 f<)r this section. Professor Baird is, however, careful to add tiiat this name was 

 constructed l)y Selys-Longchamps to accommodate the European amp/iihivs, 

 /erresfrls, &c., the skull of which is more like Pedomys austerus; and tiiis is 

 exactly so. Now, the very first and most esscnlial character of Arvicola 

 r'lpar'ius, as compared with Chilotus oiegoni, Pedomys austcms, and Pitymys 

 intiefonim, is the remarkable posterior crcscentic 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 fniipJiibiiis, 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 Ilemiotomys 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 I)e 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 

 PedoMiys. 



After coming to this conclusion, our first care, of course, was to sec what 

 European or Asiatics species our A. riparius did agree with in this particular 

 pattern of the last upper molar. Among the limited nund)er of skulls at our 

 command, in addition to A. amphihius, we fmd as follows: — A Lapland skull, 

 labeled ^'ratticeps" (No. 105G), has on the back up|)er molar an anterior loop, 



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