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.
Figs. 32-34. Lateral views of the left ear ossicles (articulated) of the Recent genera of the subfamily Zapodinae. × 20.
Fig. 32. Eozapus s. vicinus, adult, male, No. 240762 USNM, Lanchow, Kansu, China.
Fig. 33. Zapus p. princeps, adult, male, No. 32858 KU, Medicine Wheel Ranch, 28 mi. E Lovell, Big Horn County, Wyoming.
Fig. 34. Napaeozapus i. insignis, adult, male, No. 9544 KU, 3 mi. W Base Station, Coos County, New Hampshire.
Distribution of and Speciation in the Genus Zapus