420 University of California Publications in Zoology [Vol.12 



lacking; (b) dimensions of the sealed portions of the tails were not 

 usually taken in the field, and absolutely accurate measurement is 

 impossible in dry skins on account of their shrinking and crinkling ; 

 (c) the irregularity of their arrangement makes it difficult to avoid 

 error in counting the scale-rows transversely on the tail. 



Keeping these possibly modifying factors in mind, it is believed, 

 however, that certain general propositions may be formulated and 

 regarded as fairly dependable : 



(1) Weight and dimensions increase with age. Growth con- 

 tinues as in certain other mammals (for example, the gopher, 

 Thomomys) practically through life. 



(2) The number of scale-rows on the tail is apparently the same 

 in adults and in juvenals, the increase in size taking place through an 

 augmentation in measurements of the individual scales. 



(3) The ratio of the length of the tail vertebrae to total length 

 apparently increases with age (no. 12108 constitutes an apparent 

 exception to this statement). 



(4) There is evident a tendency for the tail to increase in width 

 somewhat more rapidly than in length. 



There is great individual variation in the ratio of the width of 

 tail to length. According to the table, the maximum of this ratio is 

 51.0 percent, minimum 31.8, indicating a variation of 19.2 percent. 

 The average of all the ratios is 41.3 percent. Although the animals 

 presenting the highest ratios are adults, there is no very clear cor- 

 relation between age and different proportional dimensions of tail. 



Two specimens (nos. 71830, 71833, loaned by the Biological Sur- 

 vey), being younger than the youngest listed in the table, have ratios 

 of 44 and 47 respectively. 



Coloration and Pelage 



On the whole, the coloration of the juvenals is very much like 

 that of the adults. There is a change toward a deepening in general 

 coloration, and a slight differentiation of color areas with increased 

 age. In the adults there tends to be a dark area (one obtains a 

 general impression of seal brown or a little paler) in the middle of 

 the back, with a lightening of coloration (varying from near hazel 

 or cinnamon-buff to chestnut) on the sides of the face, the top of 

 the head, the nape of the neck, the shoulders and the rump. The 

 pelage of the young is softer and fluffier than that of the adults. 



