r>i4 



U. S. P. R. R. EXP. AST) SURVEYS ZOOLOGY GENERAL REPORT. 



presenting features which are readily appreciable. These I shall now characterize, after 

 presenting their chief features in a tabular abstract. In this I shall include Hypudaeus, for 

 the sake of showing some of its peculiarities, as compared with those of the different groups 

 mentioned. 



ANALYSIS. 



Ears hidden, small ; margins incurved, surface naked B. 



" " margins not incurved D. 



" larger, scarcely hidden, not projecting conspicuously A. C. 



" projecting conspicuously 



Fore feet about half the length of hinder, their claws not larger than 



posterior A. B. C. 



" two-thirds the hinder, fore claws largest D. 



Soles with five tubercles B. C. D. 



" six tubercles A. 



Interparietal bone, subtruncate laterally A. B. C. 



" acute angled D. 



Second upper molar with two internal triangles A. 



" " one internal triangle B. C. D. 



Third upper molar with two external triangles and a posterior crescent... A. 



" " one exterior triangle and a posterior V or Y B. C. D. 



Anterior lower molar with three internal and two or three external closed 



triangles A. B. 



" " with two internal and one external closed triangle C. D. 



Anterior lower molar with salient angles mostly alternating A. B. C. D 



" " u opposite 



Middle lower molar with one posterior triangle and two lateral on each 



side, lateral salient angles alternating A. B. 



'* with one posterior triangle, one anterior, and one 



lateral on each side C. D. 



" with an anterior triangle, a middle, and a posterior.. 



Hypudaeus. 

 Hypudaeus. 



Hypudaeus. 

 Hypudaeus. 



Hypudaeus. 



Hypudaeus. 



In throwing the various species of the genus Arvicola (as restricted) into sections, I shall be 

 obliged, for convenience of reference, to give names to these, although I do not present them as 

 of even sub-generic value. It is, however, very possible that careful anatomical investigations 

 of the American species, which I have not now time to make, may render it expedient to adopt 

 many more sub-genera, or even genera, than are at present admitted. Whoever will undertake 

 the critical revision of the entire sub-family in all its species, whether Old World or New, and 

 show exactly their relationships and limits, will do what, more than any other labor, will tend 

 to relieve the study of the smaller mammals from many of its embarrassments. There is no 

 other group of Rodents in which the species are so difficult to define, nor is there any in which 

 the representatives froui the two continents approach each other so closely. I have before me 

 a large proportion of all the species known, but the limited time at present at my command will 

 not allow me to attempt any monographic investigations. Even of the species of American 

 Arvicolae I shall attempt no critical revision, but satisfy myself with giving those which appear 



