﻿July, 1896.] 



SUBGENUS MICTOMYS. 



35 



General remarks. — The characters distinguishing the subgenera Syn- 

 aptomys and Mictomys are discussed under the latter. 



Three species of true Synaptomys are now known: iS. cooperi Baird, 

 JS.fatuus Bangs, and S. helaletes Merriam.^ 



Subgenus MICTOMYS True. 



1894. Mictomys True, Proc. U. S. Nat. Mus., XVII, No. 999, p. 242. Advance sheet, 

 April 26, 1894 (full genus). Type Mictomys innuitus True. 



1896. Mictomys Merriam, Proc. Biol. Soc. Washington, X, p. 57, March 19, 1896 

 (subgenus). 



Geographic distribution of type species. — Synaptomys innuitus is known 

 from the type locality only, Fort Ohimo, Ungava, Labrador. . 



Geographic distribution of subgenus, — Hudsonian zone from Labrador 

 to Alaska, south to northern California. 



Essential characters : 



Rostrum slender. 



Palate not as in true Microtus. 



Mandibular molars without closed triangles on outer side. 

 MammjB 8. 



Skull. — The skull of Mictomys is in general much like that of Synap- 

 tomys proper, but the whole rostral part (including incisors) is dispro- 



portionally slender and weak 

 (fig. 9, and PI. I, fig. 12). The 

 bony palate is formed on the same 

 plan as that of true Synaptomys 

 or of Microtus proper, but differs 

 from both of these in the prolon- 

 gation of the median ridge as a 

 spine projecting into the inter- 

 pterygoid fossa. 



The x>terygoids are usually 

 longer and more slender than in 



Synaptomys, and the hamular pro- fig. g.-a. Svnaptomys helaUtes,- l. Synaptomys 



cesses less strongly bent outward. wrangeu. 



Teeth. — The incisors in Mictomys are much smaller in proportion to 

 the size of the skull than in the subgenus Synaptomys. The grooves 

 in the upi)er incisors are usually nearer the middle of the tooth, and 

 less well defined than in true Synaptomys. 



The maxillary teeth (fig. 10) are exactly as in the subgenus Synap- 

 tomys. The lower molars, however, differ from those of true Synaptomys 

 in the absence of reentrant angles on the outer borders of all but the 

 hindermost. Even in this the reentrant angle is never deep enough to 

 isolate an outer triangle. 



Mammw. — In the type of Synaptomys innuitus there are eight mammae, 

 two^more than have been recorded in Synaptomys jDroper. Whether 



1 See Merriam, Proc. Biol. Soc. Washington, X, p. 57, 1896. 



