94 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 



HESPEROMYS (VESPERIMUS) MICHIGANENSIS (A. & B.) Wagn. 



Michigan Mouse. 



Mux micMganenais, Aud. & Bach., Journ. Acad. Nat. Sei. Phila. viii, ii, 1842, 304; Q. N. A. iii, 1854, 326. 

 nenperomys miohiganensia, Wagnek, Wieg. Archiv, 1843, ii, 51. — Baiud, M. N. A. 1857, 476. 

 Ilrspermnij* ( l'< sp< rimus) michiganensis, Coues, Proc. Acad. Nat. Sei. Phila. 1874, 180. 

 U«« bairdii, Hoy & Kennicott, U. S. Patent-Office Rep. Agric. for 1850 (1857), 92, pi. xi. 



Diagnosis. — H. minimus, (2^—3-poll.) auriculis parvis, pedibus brevibus 

 vli~4 P"H)< caudd truncum sine capite subcequante (If— 2-poll.) ; supra cum pedi- 

 bus subjulvt's. ■end brunneus, plaga dorsali obscuriore ; infra caudidus. 



Very small mouse, yellowish-brown above, with a broad dorsal stripe of 

 sooty-brown, below pure white; feet not entirely white, as usual in leucopus ; 

 tail bicolor. Rarely 3 inches or more long; hind foot never exceeding 0.75, 

 often much shorter; ears \, or less, high; tail about equal to the trunk 

 without the head. 



Habitat. — Upper Mississippi Valley ; especially Illinois, Michigan, and 

 Wisconsin. Kansas. 



Without speculating upon the probable derivation by actual descent of 

 this species from H. leucopus, we will rest upon the fact that here we have 

 an animal positively distinct from leucopus. The differentiation from a com- 

 mon stock has proceeded so far that the connecting links, if any once existed, 

 are broken or at least concealed. Out of a considerable number of speci- 

 mens (see table below), there is not a single one that is not distinguishable 

 on sight from leucopus This, if not more remarkable, is, at any rate, the moie 

 interesting and instructive, since the true leucopus abundantly inhabits the 

 regions where, michiganensis occurs. As far as is known at present, michlga- 

 nensis is one of the most restricted of our species in geographical distribution, 

 being nearly confined to the valley of the Upper Mississippi. This, probably, is 

 the reason why it adheres so faithfully to one particular style ; and should it ever 

 become dispersed over an area large enough to bring different individuals 

 under decidedly different climatic and other influences, a divergence and varia- 

 tion would undoubtedly ensue. The only sign of such possible or probable 

 differentiation at present is represented by what has been called Mus ''bairdii" 

 by Hoy and Kennicott. These excellent naturalists were unquestionably 

 wrong in supposing a distinction of species here. Dr. Hoy expressed the 

 whole thing in a nutshell when he wrote to Professor Baird : — "One thing is 

 certain — we find one species in the oak openings, while the other is confined 



