﻿68 



NORTH AMERICAN FAUNA. 



[No. 12. 



nine. Siuce this tootli in tbe Americau species lias the same structure 

 as ill Alicrotus arvalis, no special description is necessary. 



Bach upper molar. — In the typical species the last upper molar has 

 an anterior transverse loop, a closed triangle on each side, and a very 

 short, simple terminal loop. With these loops are associated six salient 

 angles, two on each of the terminal loops and one on each closed triangle. 

 Rarely the posterior terminal loop is reduced by the isolation of the 

 outer basal angle as a third closed triangle, but this seldom happens, 

 while the resulting form of tooth is quite different from that found in 

 any member of the subgenus Microtus except the aberrant M. nivalis. In 

 the American species this tooth is formed exactly as in JMicrotus arvalis. 



Mammcc. — There are eight inammcC in Arvicola^ as in Microtus. 



Feet. — In Arvicola the soles are very sparsely haired or almost naked 

 between the tubercles and the heel. 



The tubercles are only five in number, as the small one which in 

 Microtus lies midway between the large proximal tubercle and the base 

 of the fifth toe is absent. Claws moderately developed, those on hind 

 feet slightly the larger. 



Fur. — The fur is close, dense, and long, the under fur especially thick 

 and woolly. It thus resembles the fur of Weojiher^ though the modifica- 

 tion is not carried so far as in the latter. 



Miscellaneous characters. — The species of Arvicola are provided with 

 a large musk gland on each side of the abdomen. These glands lie 

 immediately in front of the hind legs and are very conspicuous in alco- 

 holic specimens. In a half-grown male Microtus terrestris from St. 

 Petersburg, Russia, each gland is 13 mm. long by 6 mm. wide. They 

 are regularly oval in outline, the long axis parallel wath the long axis 

 of the body. The surface, which is slightly raised above that of the 

 surrounding skin, is closely and irregularly wrinkled, and has much 

 the appearance of very finely honeycombed tripe. Each gland bears a 

 sprinkliiig of fine hairs much shorter than the fur, but at first sight 

 aiipears to be naked. In dried skins the positions of the glands are 

 indicated by tufts of grease-soaked fur. 



General remarks. — The subgenus Arvicola is distinguished from all 

 other groups with similar enamel pattern or with like numbers of- 

 mammae and foot pads by the presence of the large glandular masses 

 on the sides of the body. The species are all water rats, and, with the 

 exception of Microtus {Neofiher) alleni, they considerably exceed the 

 other members of the genus in size. 



Although this subgenus is now for the first time recorded from 

 America, at least three species of Arvicola inhabiting the western 

 United States have been described within the past five -years. These 

 are Microtus macropus (Merriam), M. arvicoloides (Rhoads), and M. 

 principalis Rhoads. Microtus macropus was supposed to be *'one of 

 the w^esterii members of the subgenus or section Mynomes,^^ that is, a 

 tetramerodont Microtus.^ Microtus arvicoloides was made by its descri- 



1 North American Fauna No. 5. p. 60, July, 1891. 



