From Zapus hudsonius intermedius, Z. h. americanus differs as follows: Smaller; color brighter, more ochraceous, less yellow; braincase relatively narrower; auditory bullae usually smaller; incisive foramina broader; inferior ramus of zygomatic process of maxillary usually with median projection; interorbital region averaging broader.
For comparison with Zapus hudsonius acadicus see account of that subspecies.
Remarks.—Intergradation with Zapus hudsonius acadicus occurs in southeastern New York as indicated by a series of 25 specimens from Peterboro. They resemble Z. h. acadicus in width of the mastoid region and relatively longer tooth-row, but in the size and shape of the auditory bullae, width of the pterygoid fossae, and lighter, brighter, color of the sides they are more nearly like Z. h. americanus to which they are here referred.
Intergradation between Z. h. americanus and Z. h. acadicus is indicated also by specimens from Lawyersville and Schoharie, New York. In animals from both localities the length of the zygomata and the breadth of the mastoid region are more nearly as in Z. h. acadicus, but in size and shape of the auditory bullae, over-all length of the skull, color of the ears, and general color of the pelage they are more nearly like Z. h. americanus to which they are here referred.
Specimens from western Pennsylvania, judged to be Z. h. hudsonius by Preble (1899:17), and those from northwestern Pennsylvania and northeastern Ohio, allocated to Z. h. hardyi (= acadicus) by Bole and Moulthrop (1942:165), are more nearly like Z. h. americanus in size and shape of the auditory bullae, short zygomata, relatively narrow mastoid region, and color of pelage.
Specimens from the lower peninsula of Michigan, northeastern Indiana, and northwestern Ohio, described by Bole and Moulthrop (op. cit.:168) as belonging to a new subspecies (Zapus hudsonius brevipes), are to me indistinguishable from most specimens of Z. h. americanus. The characters which Bole and Moulthrop (loc. cit.) ascribe to Z. h. brevipes—color bright Ochraceous-Buff, tail and hind feet short, and skull narrow—are also those of Z. h. americanus.
Specimens from various localities in southeastern Ohio, all within the range ascribed by Bole and Moulthrop (op. cit.:169) to Zapus hudsonius rafinesquei, are indistinguishable from specimens of Z. h. americanus from eastern Tennessee, West Virginia, North Carolina, and Maryland. Zapus hudsonius rafinesquei (at least that part from southeastern Ohio) is indistinguishable from Z. h. americanus and therefore is synonymized under Z. h. americanus.
Specimens from Lagrange County, Indiana, show intergradation between Zapus hudsonius intermedius and Z. h. americanus in the color of the pelage but are more nearly like Z. h. americanus to which they are here referred. One from Porter County, Indiana, is more nearly like Z. h. intermedius in size and shape of the bullae and in breadth of the pterygoid fossae but in color and degree of lateral bowing of the zygomata is better placed with Z. h. americanus.
Z. h. americanus is a wide ranging subspecies. Animals at the northern periphery of the range (lower peninsula of Michigan to the west and southeastern Massachusetts to the east) are largest and darkest; to the southward there is a progressive reduction in size and a change to a lighter, brighter color. Animals from Maryland, Virginia, and North Carolina are more nearly average representatives of the subspecies than are those from the region of the type locality.