1918] Taylor: Revision of the Rodent Genus Aplodontia 439 
Greatest width of interpterygoid fossa: at expansion of fossa 
immediately back of hard palate. 
Mastoid width of cranium: greatest inclusive measurement taken 
outside of mastoid processes. 
Alveolar length superior cheek teeth: most anterior point on 
alveolus of premolar four to most posterior point on alveolus of molar 
three. 
Distance between infraorbital foramina: measured on ventral 
surface of skull. 
Mandible, transversely across angular process: greatest dimension 
along axis of process, nearly at right angles to axis of mandible itself. 
Greatest length of mandible: most posterior point on articular 
condyle of mandible to most anterior point on alveolus of incisor. 
External measurements are ordinarily taken as given below. The 
short and well-haired tail of Aplodontia makes the determination of 
the total length a matter of some difficulty, in consequence of which 
this measurement has sometimes been taken on the skinned body 
instead of as specified below. 
Total length, on unskinned body stretched out lengthwise, most 
anterior point on cartilage of nose to tip of tail, exclusive of hairs. 
Hind foot, heel to tip of longest claw. 
B. VARIATION IN APLODONTIA 
. 1. Ack VARIATION 
As the aplodontia grow older the soft gray pelage of the young 
animal becomes less soft and more brownish or blackish, a ventral 
brown wash may appear in the adult though seldom in evidence in 
the young, and the numerous white-tipped hairs which stand out so 
conspicuously in the pelage of the juvenal become obscured. That 
specific differentiation takes place early is indicated in several of 
the subspecies, in none more strikingly, however, than in Aplodontia 
rufa nigra, in which the black coloration of the adult is noted in 
animals of the vear taken in July. 
The following tendencies may be observed in the crania as maturity 
approaches. In dorsal view (pl. 25) all the sutures but those bound- 
ing the nasal bones laterally tend to disappear; the interorbital con- 
striction tends to grow narrower, proportionally and absolutely ; the 
temporal lines or ridges become accentuated and approach one another, 
