1918] Taylor: Revision of the Rodent Genus Aplodontia 443 
teeth are subject to greater variation, while the length of the incisive 
foramina, greatest width of interpterygoid fossa, and distance between 
infraorbital foramina are subject to a considerable range of varia- 
tion. It is doubtless not an accident that the measurements which, 
in the nature of the case, can be taken with greatest precision, as 
basilar length and mastoid width, appear to be subject to less varia- 
tion than those which are more difficult to take with entire accuracy. 
The outline of the nasals is fairly uniform in the entire series, the 
small variation observable being in the condition of the shallow embay- 
ment in the lateral outline of the nasals posteriorly, and in their 
relative breadth anteriorly and posteriorly. The width of the inter- 
Figs. B-D. Posterior view crania of Aplodontia rufa pacifica, to illustrate 
variation in relation of parts. X 1. Fig. B, no. 89309, Biol. Surv. Coll.; fig. C, 
no. 9052, Field Mus. Nat. Hist.; fig. D, no. 77371, Biol. Surv. Coll. 
orbital constriction is highly variable, not only in animals of different 
ages, but also in animals of apparently similar age. In certain speci- 
mens a little rounded notch is developed in the lachrymal region, on 
the anterior border of the orbit. In others, this notch is feebly 
developed or lacking entirely. The degree of development and ap- 
proach of the temporal lines or ridges is variable. In some specimens 
the ridges are weakly developed and comparatively far apart, in 
others strongly developed and much closer together. The weight of 
