1916] Taylor: Beavers of Western North America 457 



wiek examples. Zygomata not quite so square anteriorly in frondator 

 as in specimens of belugae from Cook Inlet. Also distance from 

 dorsal outline of foramen magnum to inion less (see fig. G, p. 431; 

 frondator, no. 35946, 18.0 mm., 60354, 18.9; belugae, no. 4347, 20.5 

 mm., 4225, 22.4, 4224, 22.7) ; teeth narrower on average (see table of 

 measurements, p. 428). 



Nasal outline, condition of median process in the interpterygoid 

 fossa, and certain skull dimensions, show the affinities of frondator 

 to be with canadensis rather than with either subauratus or pacificus. 



Outline of the History op the Beavers 



The Castoridae and Castoroididae are probably derived from that 

 primitive Eocene rodent stock represented in the family Ischy- 

 romyidae. Matthew (1910, p. 67) has outlined their possible rela- 

 tionships to this family, postulating their descent from the North 

 American Paramys, an ancient squirrel-like form which with its con- 

 temporary relatives was perhaps ancestral to most, possibly to all, 

 the members of the great order Rodentia. It has so far been impos- 

 sible definitely to work out whether Plesiarctomys from the French 

 Upper Eocene is congeneric with Paramys or distinct from the Ameri- 

 can genus. 



According to Matthew's scheme, Paramys gave rise to Sciuravus 

 of the North American Eocene, which in its turn is tentatively placed 

 as the ancestor, on the one hand, of the stock which gave rise to 

 Eutypomys of North American Middle Oligocene, and on the other, 

 of that from which the important genus Steneofiber is derived. These 

 two genera are the earliest which are referred by Osborn (1910. 

 p. 535) to the Castoridae. 



Concerning Eutypomys very little is known. Osborn refers to it 

 as a ground-squirrel or semi-cursorial type, and suggests that it may 

 be related to the Pteromyinae. It seems to be of little importance 

 to the phylogeny of beavers. On the contrary, Steneofiber assumes 

 a place of great importance in this phylogeny, being the supposed 

 ancestor of most of the later genera belonging to the beaver family. 



Steneofiber, which was about the size of a marmot, appears first 

 in the Middle Oligocene (Stampian) of Europe, also throughout the 

 Upper Oligocene of North America, being abundant and characteristic 

 in the Upper John Day of Oregon. Evidently it became extinct soon 

 after, for it has not been discovered in later deposits. At least by 



