North American Jumping Mice (Genus Zapus) - Philip H. Krutzsch

Figs. 22-31. Dorsal and lateral views of the bacula of the Recent genera (and species of the genus Zapus) of the subfamily Zapodinae. × 10.

Figs. 22 and 27. Eozapus setchuanus (after Vinogradov, 1925:585).

Figs. 23 and 28. Zapus t. trinotatus, adult, No. 94596 MVZ, 1¹/₄ mi. ENE Amboy, 350 ft., Clark County, Washington.

Figs. 24 and 29. Zapus p. princeps, adult, No. 20870 KU, 3 mi. S Ward, Boulder County, Colorado.

Figs. 25 and 30. Zapus h. pallidus, adult, No. 22954 KU, 4 mi. N, 1³/₄ mi. E Lawrence, Douglas County, Kansas.

Figs. 26 and 31. Napaeozapus i. insignis, adult, No. 41110 KU, Shutsburg Rd., at Roaring Creek, 600 ft., Franklin County, Massachusetts.

Ear ossicles.—The auditory ossicles are of three types which differ only slightly. These ossicles possibly are more conservative than some other structures because the ossicles are not so much affected by the molding influence of the environment.

Instances of variation in the auditory region in mammals in general are small, even at the family level; therefore, these differences in the subfamily Zapodinae are offered as additional support for recognizing Eozapus, Zapus, and Napaeozapus as distinct genera. The distinctive features are chiefly in the malleus and incus; the stapes, however, differs slightly and, therefore, it too is described (see [figs. 32-34]).

In Eozapus the head of the malleus is narrow, oblong, and rounded dorsally and attaches to the body by a long, slender, abruptly recurved neck. The body is weakly pointed ventrally and rounded dorsally. A beaklike manubrium malleus composed of anterior projecting external and internal spines extends from the body to the tympanum. The incus has a dorsally rounded body with an anterior downward snoutlike projection with which the malleus articulates. The short limb of the incus is broad basally and narrows somewhat distally. The long limb is narrow and its articulating lenticular process is a flat circular structure. The limbs of the stapes are wide-spread and heavy. The neck is short and wide with a large circular articulating surface.