Description.—Size medium; back from near Ochraceous-Tawny to near Yellow-Ocher with heavy admixture of black-tipped hair, the dorsal band distinct against color of sides; sides lighter than back and from near Cinnamon-Buff to near Ochraceous-Buff lined with black-tipped hair; lateral line usually [433] faintly marked but sometimes distinct and clear Warm-Buff; underparts white, sometimes suffused with color of sides; tail distinctly bicolored, brownish-black above and yellowish-white to grayish-white below; ears dark, edged with color of sides; feet grayish-white above; pterygoid fossae relatively narrow; zygomata relatively long and broad; auditory bullae relatively narrow, usually with depression on anterior surface; mastoid region relatively narrow; inferior arm of zygomatic process of maxillary relatively narrow.
Comparisons.—From Zapus hudsonius canadensis, Z. h. acadicus differs in: Size averaging larger; upper parts usually less brownish and more ochraceous, sides and flanks being more ochraceous and less yellowish; zygomata relatively longer; pterygoid fossae relatively narrower; auditory bullae relatively narrower and usually with depression on anterior surface.
From Zapus hudsonius americanus, Z. h. acadicus differs as follows: Size larger; color darker on upper parts, flanks duller (less ochraceous); underparts white, much less frequently suffused with color of sides; ears dark, usually without flecks of ochraceous; general appearance of pelage not so brightly colored; zygomata longer; condylobasal length greater; mastoid region relatively broader; bullae larger, more inflated and usually with depression on anterior surface; maxillary tooth-row relatively longer.
For comparison with Zapus hudsonius ladas see account of that subspecies.
Remarks.—Specimens from various localities in Nova Scotia, Prince Edward Island and New Brunswick are essentially similar. Anderson (1942:38) revived the name Z. h. acadicus for jumping mice from these areas, correctly considering them to be distinct from Z. h. canadensis, the geographic race immediately to the west.
In the size and shape of the auditory bullae, length of the zygomata, breadth of the pterygoid fossae, and general color of the pelage the populations from Nova Scotia and New Brunswick are essentially indistinguishable from material of Zapus hudsonius hardyi from Maine. Thus, Z. h. hardyi must fall as a synonym of the earlier proposed name Z. h. acadicus.
Bole and Moulthrop (1942:165) applied the name Z. h. hardyi (= acadicus) to the mice inhabiting a large area from coastal Maine and central New Hampshire through southern New England, New York, northwestern Pennsylvania, and northeastern Ohio. I agree with Bole and Moulthrop (loc. cit.) that the population of Zapus hudsonius from Maine, New Hampshire, west-central and northern New England are different from neighboring subspecies and are referable to Z. h. acadicus, but find that material from extreme southern Massachusetts, Connecticut, southern New York, northwestern Pennsylvania, and northeastern Ohio is best referred to Zapus hudsonius americanus (see account of that subspecies).
Intergradation between Z. h. americanus and Z. h. acadicus is indicated by specimens from Berlin, Rensselaer County, New York. In color of ears, length of zygomata, and size and shape of the incisive foramina these specimens are more nearly like Z. h. americanus but in size and shape of the auditory bullae, breadth of the mastoid region, and general appearance of the pelage they are more nearly like Z. h. acadicus and are here referred to acadicus. Specimens from Glenville, Schenectady County, New York, are intermediate in cranial characters between Z. h. americanus and Z. h. acadicus but in color are best referred to the latter. Specimens from 1 mi. S Ayer, Worchester County, Massachusetts, are like Z. h. americanus in their short zygomata, narrow mastoid region and suffusion of the underparts; nevertheless, in the shape of the auditory bullae, breadth of the pterygoid fossae, and greater condylobasal length the specimens are more nearly like Z. h. acadicus which they are here considered to be. Animals from Essex and Wilmington, Essex County, Massachusetts, are like Z. h. americanus in external size and in the size and shape of the auditory bullae; but they are more nearly like Z. h. acadicus in most cranial characters and in the general color of the pelage and are here assigned to Z. h. acadicus.
Specimens from Keene Valley, Essex County, New York, considered by Batchelder (1899:4) to be Z. h. canadensis, are in color, length of the zygomata, and size and shape of the auditory bullae more nearly like Z. h. acadicus to which subspecies they are here assigned. A specimen from Orwell, Addison County, Vermont, that Batchelder (op. cit.:5) referred to Z. h. canadensis is more nearly like Z. h. acadicus in the shape of the auditory bullae, length of the zygomata, and color of the pelage, and is here referred to Z. h. acadicus. Specimens from western New Brunswick, referred to Z. h. canadensis by Anderson (1942:37), are more nearly like Z. h. acadicus. Specimens from Ste. Anne des Monts, Gaspé Peninsula, Quebec, are intermediate between Z. h. canadensis and Z. h. acadicus in color and size and also in the shape of the auditory bullae but are best referred to Z. h. acadicus.
Zapus hudsonius acadicus as here understood is a relatively wide-ranging subspecies. Populations at the southern periphery of its range are difficult to separate from populations at the northern periphery of the range of Z. h. americanus. These two geographic races represent opposite extremes of a clinal gradient and, as would be expected, geographic intermediates are morphologically similar.