﻿July, 189P.] 



SUBGENUS MICROTUS 



65 



extended and cut by a reentrant angle on the lingual side, so that the 

 crescent is modified into the form of a rude E. Occasionally the ante- 

 rior extremity of the crescent is isolated as a second inner triangle. 

 Tlie convex side of the crescent may develop a more or less prominent 

 salient angle. This condition is normal in Microtus rattieeps and Micro- 

 tus chrotorrhimis, but occurs also in other species. In the aberrant 

 ARcrotus nivalis the structure of this tooth is simplified so that it is 

 essentially as in Arvieola^ Pedomys^ and PitymijH. 



Other teeth. — The first and second upper molars contain each an ante- 

 rior transverse loop and, respectively, threfi and t^^ closed triangles. 

 In 2Iicrotus agrestis^ 21. Hilxlximensis^ 21. perinHylvanicus^ 21. terrecnovcv, 

 and 21. aztecus the inner edge of m2is produced into a conspicuous 

 loop, which frequently becomes isolated, so as to form a closed triangle 

 about half the size of the others. The European species with m2 

 formed in this way have been T)laced in a subgenus called Agricola or 

 Sylvicola, while the American species have been referred to 2Iynomes 

 in a restricted sense. The American species with m 2 exactly as in 

 Microtus arvalis have received the name Tetramerodon. While the 

 name Tetramerodon can not be used in a subgeneric sense, it is fre- 

 quently convenient to sx^eak of the voles with the enamel pattern of 

 If. arvalis as the tetramerodon t species to distinguish them from their 

 pentamerodont allies. In 2licrotus silxldmensis a supplemental triangle 

 is developed in m 1 as well as in m 2. On account of this peculiarity the 

 animal has been made the type of the genus or subgenus ^Xeodon.^ 

 Keither Neodon nor Agricola are worthy of recognition as subgenera 

 distinct from 2[icrotus. Their characters are of trifling importance, 

 while in other species of 2licrotus (as, for instance, 21. nivalis, 21. giien- 

 theri, and occasionally 2f. pennsylvanicus) intermediate conditions can 

 be found. 



2Iamma\ — In the subgenus 2[icrotus the manmne are always eight, 

 four pectoral and four inguinal. Xo exceptions to this number are 

 known. 



Feet. — There are six turbercles on the sole. Five of these are always 

 well developed, but the sixth is variable in size, being especially large 

 in 21. ratticeps. The sole is always moderately hairy from heel to 

 tubercles. It is never densely furred as in Fhaiomys or naked as in 

 Xeofiher. The claws on all four feet are moderately developed, those 

 on the hind feet always slightly larger than those on the front feet, 

 the latter never specially develoj)ed for digging (cf. Fitymys). 



Fur. — The fur is moderately full and soft, neither long and silky as 

 in Fhaiomys nov dense and mole-like as in Fitymys. 



General remarlis. — Tlie subgenus 2Iicrotus needs com[)arison with the 

 groups having normal or very slightly abnormal palates: Arvicola, 

 Fedomys, Fitymys, Ghllotus, Fhaiomys, and Lagurus. From all the 

 others it differs too widely to give rise to confusion. Lagurus is dis- 

 tinguished from 21icrotus by the tightly closed triangles in the jjosterior 

 16933— Xo. 12 5 



