444 University of California Publications in Zoology [Vol. 12 



not extend posteriorly of a line connecting antorbital tubercles in 

 either leucodonta or pacificus. 



The lateral ridge on rostrum is more prominent in leucodonta 

 than in pacificus. 



The hamular blades are broad in leucodonta (see fig. F, p. 430) ; 

 narrow in the Lake Cushman examples, and tending to show an 

 intermediate condition in Castor c. belugae from Stuart Lake. 

 Broad in two juvenals from Fisher, Oregon (nos. 136605, 136606) 



as well as in a young topotype skull of pacificus from Lake Kr he- 



lus, Washington. Measurements: Castor c. leucodonta, Vancouver 

 Island, no. 12107, 4.4 mm.; 12101, 4.6; 12111, 4.8; Castor c. paci- 

 ficus, Lake Cushman, Washington, no. 71S14, 2.2 mm.; 87629, 2.8; 

 Castor c. belugae, Stuart Lake, British Columbia, no. 77155, 3.1 

 mm. ; 77150, 3.2 ; the two juvenals from Fisher, Oregon : no. 136605, 

 4.3 mm. ; 136606, 3.8 ; the young topotype cranium of Castor c. 

 pacificus: no. 126190, 4.3. In leucodonta this character varies little 

 with age. That it may do so in other subspecies, however, is indi- 

 cated by the fact that a young specimen of undoubted Castor caua- 

 densis frondator (no. 60355), has broad-bladed hamulars, while an 

 older specimen of the same subspecies (no. 35946) has narrower 

 ones. 



The occipital crest is flattened in leucodonta, upright in all but 

 one of the specimens of belugae from Stuart Lake, British Colum- 

 bia, and in a majority of those of pacificus from Lake Cushman, 

 Washington. The disparity between pacificus and leucodonta in 

 this respect is shown by the measurements given in the tables (p. 445. 

 and opp. p. 426) under the heading "most dorsal point on the outline 

 of foramen magnum to occipital crest on the median line" (see fig. G, 

 p. 431). To make the comparison fair, only the three oldest crania 

 of leucodonta (nos. 12111, 12101 and 12107) should be taken into 

 account. 



The foramen magnum is similar in general shape. Range of 

 the ratio of vertical diameter of foramen magnum to basilar length : 

 In belugae, with old and young crania represented, 9.8 percent to 

 14 per cent; in pacificus, with adult skulls only represented, 9.0 

 per cent to 12.5 per cent; in leucodonta, with old and very young 

 skulls represented, 11.3 per cent to 17.7 per cent. 



The specimens from the mainland of British Columbia and 

 Washington are clearly set off from Castor canadensis canadensis 

 and its close relative C. c. michiganensis through the characters of 



