Internal Characters 
Viewed from the side, the skull on top is remarkably flat and 
parallel with the base; the lower edge of the occiput is horizontal 
and perpendicular to the longitudinal axis of the skull. The palate 
continues in a straight horizontal line from the incisors to the ends 
of the pterygoids. 
Seen from above, the skull shows a nearly equilateral triangle. 
The rostrum is stout and sloping. The zygomatic process curves outward 
and backward, then contracts to the jugal forming a deep arch. The 
auditory bulla is flack-shaped. The massester muscle originates prin- 
cipally beneath the infraorbital foramen and no part of the muscle 
reaches the rostrum, The skull is also remarkable in that it lacks 
postorbital processes. 
The generic word Aplodontia comes from two Greek words: haploos 
(simplex) and odontos (teeth), meaning simple teeth. The cheek teeth 
are modified hypsodont. All the molars are rootless and prismatic. 
The pattern of the molar crowns is simple, except the anteriormost 
upper premolar which is a small simple cylinder lying obliquely against 
the anterolingual corner of the second premolar. The second premolar 
and three molars are set obliquely outward, decreasing in size from the 
second premolar to the last molar, Each tooth has a prominent style on 
the labial side of the upper teeth and on the lingual side of the lower 
teeth. The anterior faces of the stout incisors are smooth, convex in 
both directions, and their sides bevelled. Dental formula for the 
mountain beaver is: 
I, c, Pe, Ms x 2 = 22 (fig. 3). 
REPRODUCTION 
The mountain beaver is monestrous, has a low reproductive rate, 
and usually does not bear young before the second year. The mating 
season usually begins in late February or early March. Pfeiffer (1958), 
who studied reproduction in the female mountain beaver, found that the 
estrous female has hypertrophied nipples, a greatly swollen vulva, and 
an enlarged uterus and cervix. 
By estimating age from the degree of closure of the distal epi- 
physeal femoral suture and measuring estrous uteri, Pfeiffer classified 
four age classes of females, He classified them as: 
I. Short yearlings (all under 1 year) 
II. Long yearlings (between first and second year), nulliparous 
III. End of second year or early in their third year between 
December and July 
IV. No sutures, hence full grown and parous females. 
The majority of parous females were over 2 years old. 
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