l. r )0 MONOGRAPHS OF NOIiTn AMERICAN RODEXTIA. 



various sections of the genus that it is quite worth while to recognize. The 

 names Psammomys LcC. {nee Rtipp.), Pitymys McMurt, and Pinemys Less, 

 are mere synonyms, all having been based on the same animal (pinetorum), 

 and two of them being simply amends for LeConte's mistake in taking 

 Vsammomys Riipp. for his Arvicola pinetorum. " Microtiis" Selys. appears 

 to he about the same thing as Pitymys. Hemiotomys Selys. {nee Baird) goes 

 to the European amphibius. Myonomes Raf. and Pedomys and C/iiloti/s Baird 

 are tenahle suhgeneric names for particular American groups. Doubtless 

 tlicre are some other generic or suhgeneric names that have not come to our 

 notice. 



In proceeding now to define Arvicola, we may premise that our diag- 

 nosis of the genus will simply he equivalent to such restriction of the char- 

 acters of the subfamily Arvicolince as will exclude the Lemmings, the 

 remarkable Synaptomys of Baird, and the rooted-molar group (Evotomys nob. 

 — Hypudceus Keys. Bias. Bd. nee 111.). Some of the more boreal Arvicola do 

 indeed closely approximate to the Lemmings in the shortness of their ears, tail, 

 and feet, and in the mollipilose pelage; but the radical differences in dentition 

 are never, so far as we know, obscured. It may be that there are some 

 species of Arvicolince that require generic separation from Arvicola besides 

 those just mentioned, but none such have come to our knowledge. 



Some of the characters we are about to give are rather those of the sub- 

 family than of the genus; but the particular combination, as expressed in the 

 whole paragraph, is generally diagnostic. 



Gen. Chars. — Crowns of the (§— f, rootless, perennial, prismatic) molars 

 plane, divided into several closed islands of dentine by folds of the surround- 

 ing sheet of enamel that meet from opposite sides and fuse along the median 

 line (cf. Evotomys) ; the saliencies and reentrances of the alternating prisms 

 strong and sharp, equally so on both inner and outer sides of the molar series, 

 the profile of which is therefore equally serrate on both sides (cf. Myode*, 

 Sy?iaptoi7iys). Anterior upper molar of 5 prisms, — 1 anterior, 2 interior, 2 

 exterior. Middle upper molar of 4 (or 5) prisms, — 1 anterior, 1 interior, 2 

 exterior (the last sometimes giving oft* a supplementary postero-interior one). 

 Posterior upper molar of 4 to 7 prisms, of which the first is always anterior 

 and transverse, the last a variable treflle (C, G, U, V, Y, &c, in shape, 

 according to subgenus or species), and the intermediate ones latci-aT and alter- 

 nating. All upper molars subequal in length and breadth (cf. Myodes, Synap- 



