Microtus pennsylvanicus alcorni new subspecies
_Type._--Female, adult, skin with skull, No. 21552, Univ. Kansas, Mus. Nat. Hist., 6 mi. SW Kluane, 2550 feet elevation, Yukon Territory, Canada; 24 August 1947; obtained by J. R. Alcorn; original No. 5240.
_Range._--Extreme southwestern Yukon Territory and adjacent parts of Alaska as far south as Haines, as far north as Northway, and as far west along the Alaskan coast as Anchorage and Tyonek.
_Diagnosis._--Size large (see measurements); color of upper parts near (_l_) Brussels Brown; skull noticeably ridged; zygomatic arches heavy, rounded and relatively short; rostrum heavy; auditory bullae not greatly expanded; maxillary teeth relatively heavy and low-crowned.
_Comparisons._--From _M. p. drummondii_ (specimens from vicinity of Whitehorse, Y. T., Trutch, B. C., and Kinuso, Alberta), _M. p. alcorni_ differs as follows: Averaging larger in all measurements taken except lengths of tail and hind foot, which are the same; color of upper parts slightly paler and more gray and less brown; underparts paler; zygomatic arches heavier, rounder and shorter; skull proportionately more massive, except the auditory bullae which are less inflated; maxillary teeth heavier and lower-crowned.
From _M. p. rubidus_ (specimens from Atlin, B. C.), _M. p. alcorni_ differs as follows: Averaging larger in all cranial measurements taken except length of the maxillary tooth-row which is the same; color of upperparts more gray and less brown; underparts darker; skull longer with longer nasals and heavier zygomatic arches; skull of adult more heavily ridged.
From _M. p. admiraltiae_ (specimens from Admiralty Island), _M. p. alcorni_ differs as follows: Averaging larger in all measurements taken; color of upper parts more gray and less brown, underparts darker.
_Remarks._--_Microtus p. alcorni_ is a well-defined subspecies differing markedly from adjacent subspecies by a larger and heavier skull and broader, more rounded and heavier zygomatic arches. Characters examined in the specimens available are constant. Specimens from Haines are slightly darker than those from Kluane. An adult (No. 21534, UKMNH) from Northway has slightly more inflated auditory bullae than those from Kluane. An adult from Tyonek (No. 986, UKMNH) has richer brown upper parts. Measurements of this specimen resemble closely those of animals from Kluane, although the rostrum is noticably heavier.