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U. 8. P. R. R. EXP. A5D 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... Hypudaeus. 
Fore feet about half the length of hinder, their claws not larger than 
posterior. .A. B. C. Hypudaeus. 
“ two-thirds the hinder, fore claws largest. D. 
Soles with five tubercles. B. C. D. 
“ six tubercles. A. Hypudaeus. 
Interparietal bone, suhtruncate laterally. A. B. C. 
“ acute angled... D. Hypudaeus. 
Second upper molar with two internal triangles. A. 
“ “ one internal triangle. B. C. D. Hypudaeus. 
Third upper molar with two external triangles and a posterior crescent... A. Hypudaeus. 
“ “ 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. 
“ “ “ opposite. Hypudaeus. 
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. 
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 from the two continents approach each other so closely. I have before me 
a large proportion of all the species known, hut 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 
