422 University of California Publications in Zoology [Vol.12 



In some species there seems to be decided individual variation in 

 coloration of pelage. Of the specimens from eastern Canada at 

 hand, one (no. 4358, U. S. Nat. Mus., May 21) is in light pelage, 

 while two (nos. 174525, 174526, U. S. Nat. Mus., Sept. 25) are in dark 

 pelage. Among skins of Castor canadensis phaeus from Admiralty 

 Island, one (no. 209, Mus. Vert. Zool., May 16) is very dark, while 

 another (no. 210, Mus. Vert, Zool., June 1) is paler. Details of these 

 differences appear in the tabulations of coloration in the following 

 pages. 



Tail. — The variation in measurements and proportions of tails is 

 recorded in table I, p. 419. 



Scattered hairs, which grow from between the scales, appear 

 in the tails of the juvenals, but are generally lacking in those of the 

 adults. 



CRANIAL CHARACTERS 



(See table, opp. p. 426, and fig. B, p. 424) 



General Change 



The skull is rounded in young animals, with frontals elevated, 

 and interparietal region sloping. In adults it is more flattened, with 

 frontals not elevated and interparietal region not sloping so much. 



As growth continues, the comparatively undifferentiated skull of 

 the juvenal becomes adapted to the increased strains put upon it. the 

 sutures tend to disappear, the bones harden, processes and ridges 

 develop greatly, and there is an increase in size. 



Every bone changes somewhat in outline as the animal grows older, 

 the most evident modifications being (a) loss by the frontals of their 

 jardinier or vase-shape in outline as viewed dorsally, and their assump- 

 tion of a fleur-de-lis shape, due to encroachment of temporal ridges 

 anteriorly; (&) narrowing and antero-posterior extension of the inter- 

 parietal, giving it, in outline as viewed dorsally, an Indian-club 

 rather than a subrectangular shape; (c) change in outline of the 

 palatine as viewed ventrally, so that instead of being nearly an equi- 

 lateral triangle it is isosceles; (d) widening of the foramen magnum 

 proportionally to its height, 



Teeth 

 Milk Dentition and the Eruption of the Teeth 

 The dental formula of the beaver is I\, C&, P], MfX 2 = 20. 



