G4 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
HESPEROMYS (VESPERIMUS) MICHIGANENSIS (A. & B.) Wagn. 
Michigan Mouse. 
DraGnosis.—H. minimus, (24—3-poll.) auriculis parvis, pedibus brevibus 
(3-3-poll.), cauda truncum sine capite subaquante (19+2-poll.) ; supra cum pedi- 
bus subfulvescenti 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 /eucopus ; 
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. 
Hasirar.—Upper Mississippi Valley; especially Hlinois, 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 /eucops. 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 more 
interesting and instructive, since the true deucopus abundantly inhabits the 
regions where michiganensis occurs. As far as is known at present, michiga- 
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 
