1916] 



Taylor: Beavers of Western North America 



423 



Fig. A. Occlusal 

 surface of PA, to 

 show method of 

 taking measure- 

 ments. Approx- 

 imately natural 

 size. 



Milk premolars are braehydont, usually with three 

 well-developed roots, though in one specimen at 

 band there are two roots only. Tooth eruption is 

 as follows : milk premolar 4 ; molar 1 ; molar 2 ; 

 molar 3 ; permanent premolar 4. The order of 

 appearance of the teeth is the same on both jaws, 

 the corresponding upper and lower teeth appear- 

 ing simultaneously. 



II. MEASUREMENTS* OF TEETH OF Castor canadensis leucodonta 



GRAY, FROM VANCOUVER ISLAND, BRITISH COLUMBIA 



(All measurements in millimeters) 



*Each measurement is taken three times and the results averaged to 

 give the measurement here entered. 

 tSee figure A. 



Some Characteristics of the Permanent Dentition 



The permanent dental armature exemplifies a high degree of 

 hypsodonty. The oldest crania available to the writer have the pulp- 

 cavities of the cheek-teeth almost completely closed. In order to 

 ascertain whether there is a pronounced change in size of teeth with 

 age, all the cheek-teeth on the left-hand side, above and below, of seven 

 available skulls from Vancouver Island, were measured. The skulls 

 themselves belonged to animals of different ages. In nos. 12104, 



