﻿34 



NORTH AMERICAN FAUNA. 



[No. 12. 



Microtince in its grooved incisors. From the other lemmings it may he 

 known hy its nnmodified external form, and from the voles by the 

 characters of its molars. 



Subgenus SYNAPTOMYS Baird. 

 Synaptomys Baird, Mamm. N. Am., p. 558, 1857. Type Synapiomys cooper'i Baird. 



Geographic distribution of type species. — Boreal, Transition, and north- 

 ernmost edge of Austral zone in eastern United States and adjoining 

 British Provinces,- west to Minnesota, south to Iowa, Indiana, Ohio, 

 and Maryland. 



Geograpliic distribution of suhgenus. — Boreal zone to northern edge of 

 Lower Austral zone in eastern Canada and eastern IJnited States 5 

 west to Minnesota, south to Kansas and Virginia. 



Essential characters : 



Rostrum very heavy. 



Palate nearly as in true Microtus. 



Mandibular molars with closed triangles on outer side 

 Mammse 6. 



SMIL — The skull of true Synaptomys (fig. 9 and PI. I, fig. 13) differs 

 from that of Mictomys in the remarkably heavy rostrum and in certain 

 slight details in the form of the bony x^alate. The latter is almost 

 exactly as in typical Microtus^ the slight peculiarities in form being 

 well within the limits of variation in the latter. 



Teeth. — The incisors in true Synaptomys are, like the rostrum, exces- 

 sively strongly built. The grooves are usu- 

 ally sharply defined and placed near the 

 outer edges of the teeth. 



The maxillary teeth differ in no way from 

 those of the species of Mictomys. In the 

 riG. s.-Enamei pattern of molar molar s of the lowcr jaw, howcvcr, the outer 



tQGth. of Synaptomys cooperi. (x 5.) ^ „ , , \ , i t 



edge of each tooth is cut by a deep reen- 

 trant angle which isolates a large outer triangle (fig. 8). 



Mammce. — The number of mammae in Synaptomys has been variously 

 recorded as four and six. Dr. Coues, in his monograph of the American 

 Microtince^ states that he finds six, four i)ectoral and two inguinal, in a 

 female from Brookville, Ind.^ Quick and Butler,^ however, noted only 

 four, two pectoral and two inguinal, in specimens from the same local- 

 ity. Mr. Yernon Bailey records six mammie in a female collected for 

 the United States Department of Agriculture at Ann Arbor, Mich., 

 and I find the same number in an alcoholic specimen taken at Eogers- 

 ville, Tenn. It is x>robable that six is the normal number, and that 

 Quick and Butler overlooked the ]>osterior pair on the breast, as these 

 are smaller than the others, at least in the alcoholic specimen from 

 Tennessee. 



1 Monogr. N. Am. Rodentia, p. 236. 



2 American Naturalist, XIX, p. 114. 



