1 labial re-entrant fold.

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.

In Zapus the head of the malleus is angular with an anterior projecting point and is flattened in dorsal aspect. The neck is slender, elongate, and gently curved away from the long limb of the incus. The body is pointed dorsally and rounded ventrally, the reverse of the condition in Eozapus. There is a beaklike manubrium malleus composed of internal and external anteriorly projecting spines extending from the body to the tympanum as in Eozapus. The incus has a rounded body with a long angular limb articulating via a small lenticular process with the stapes. The short limb is narrow but does not taper distally as in Eozapus. The limbs of the stapes are relatively narrow, weak, and gently curved. The neck is longer and more slender than that of Eozapus.

In Napaeozapus the head and neck of the malleus resemble those of Zapus but are less robust. The body is more rounded dorsally, having the curved dorsal surface directed anteriorly rather than posteriorly (as in Zapus) and the lateral surface is nearly flat instead of curved as in the other genera. The manubrium resembles that of Eozapus and Zapus. The body of the incus is flattened dorsally but otherwise rounded. The long limb of the incus is angular and longer than that of Zapus. The short limb of the incus is broad at the base and tapers distally. The limbs of the stapes are narrow, weak, and abruptly curved. The neck is more slender and elongate than in Zapus.

In summary: Only the head and body of the malleus and the short and long limbs and body of the incus are sufficiently consistent within a given group to be of taxonomic importance. The similarity in the morphology of these ossicles indicates a close relationship between all three genera. Zapus and Napaeozapus resemble one another more than either resembles Eozapus. The differences recorded are constant between the described groups and, therefore, are considered to be of taxonomic significance. The differences give basis for dividing the subfamily Zapodinae into the three genera Eozapus, Zapus, and Napaeozapus.