442 University of California Publications in Zoology (Vou. 17 
poor condition, suggesting that its aberrant paleness may be due to 
disease. As with the early spring examples, the uniformity of this 
late fall series of skins is more noteworthy than the variation observed. 
Total length and length of tail vertebrae in this genus, as recorded 
by the collector, are often unreliable. This follows from the fact that 
the condition of the tail, which is very short but well-haired, makes 
accurate measurement of tail vertebrae difficult. The writer has, 
therefore, put the emphasis in this discussion on the more reliable 
cranial measurements. 
In rodent species with hypsodont teeth the age of adults is deter- 
minable not at all or only with difficulty. In such species reliance 
5 10 15 20 25 30 35 40 
= 
Length of nasals 
Width of nasals 
Length of incisive foramen 
Zygomatie width 
Greatest width interpterygoid fossa 
Mastoid width 
Alveolar length superior cheek teeth 
Distance between infraorbital foramina 
Mandible, transyersely across angular process 
Greatest length of mandible 
Fig. A. Diagram showing the percentage range of variation (figured on the 
mean) in cranial measurements of twenty-three skulls of Aplodontia rufa pacifica 
(see table of measurements page 468). 
must be placed on the condition of the sutures, the outline of certain 
bones, the form of the skull, and the development of ridges and pro- 
cesses. It follows that it is possible that an observed range in varia- 
tion assumed to be individual may really be due to age; and this 
point ought to be kept in mind through the following discussion. 
The diagram (text-fig. A) illustrative of the percentage range of 
variation shows that for specific comparisons the basilar length, length 
of nasals, width of nasals, mastoid width, and the width of the 
mandible, transversely across angular process, are the most depend- 
able characters. Zygomatie width and alveolar length superior cheek 
