170 BULLETIN NO. VII. 



most rapidly, leaving little but retrenchment for the following 

 periods. 



Eucastor was a genus of small beavers the known examples 

 of which were less than half the size of our species. There is 

 some reason to doubt if it is more than sub-generically distinct 

 from the modern form. 



Steneofiber and Palceocastor are vicarious genera, the one in 

 Europe the other in America, which may easily be identical. 



Trogonotherium contained species considerably larger than ex- 

 isting beavers. 



GENUS CASTOR. 



Feet five- toed, hind feet palmate; body stout, especially be- 

 hind; tail laterally expanded, scaly; skull somewhat as in 

 squirrels but without postorbital frontal processes. Molars 

 -, single rooted, the dentinal pulp persisting until a late per- 

 iod, triturating surface composed of enamel folds; lower jaw 

 massive, symphysis firmly closed. The genus is represented 

 at present by a single circumpolar species which may be 

 divided into two sub-species or varieties with very close rela- 

 tions. 



The very careful review of the evidence bearing on the ques- 

 tion of the relation of the two forms given in Dr. Ely's account 

 contained in Morgan's "Beaver and His Works", makes a dis- 

 cussion of the matter unnecessary. Certain constant but min- 

 ute differences in osteology and the nature of the secretion are 

 demonstrated, while the importance to be admitted for them 

 must be a matter of opinion. 



Castor flb^r LINN. 



BEAVER. 



Body thick and heavy, with the greatest diameter near the 

 hips, length about 30 inches; tail, 10 inches; the head is broad, 

 depressed; nostrils lateral in a naked muffle; ears nearly hidden 

 in the fur, rounded; the fore feet are small and are not used in 

 swimming; hind feet broad, webbed, second toe often with a 

 double claw; color, reddish-brown, darker in winter. The long 

 hairs are coarse and glossy, the under pelage soft, fluffy and of 

 a grayish color. Melanism is common northward, albinoes not 

 being rare. The beaver is the largest North American rodent; 

 indeed, with the single exception of the capybara, it is the 



