1916] Taylor: Beavers of Western North Aim run 4'J."> 



1210S, and 12103 the permanent premolar has hardly become func- 

 tional. No. 12107 is the skull of a comparatively old adult, as shown 

 by its dimensions and degree of development. To reduce the margin 

 of error, each dimension was carefully taken three times, and the 

 results averaged, in case they did not exactly agree (see table II). 



It is apparent, from the data of the table of measurements, that 

 all the cheek-teeth first increase in size with age, then undergo a 

 slight absolute or proportional decrease. It should be noted that the 

 initial increase is of greater magnitude than is the later decrease. 

 The tooth pierces the gum, increases in size up to a certain point, 

 then decreases slightly . Since the oldest skulls at hand, without 

 exception, have the longest maxillary tooth-rows, the decrease in 

 tooth dimension late in life is not sufficient to affect the validity of the 

 "length of the maxillary tooth-row" as a comparative measurement. 



The table of measurements indicates that there are no important 

 differences in the relation of the antero-posterior to the transverse 

 diameter of the teeth in crania of different ages. One notes that the 

 relation of the longitudinal or antero-posterior diameter of the teeth 

 to the transverse is more variable in the superior teeth than in the 

 inferior, in which latter the antero-posterior diameter exceeds the 

 transverse in nearly all cases. 



A number of possibly significant conclusions are derivable from 

 the tables of measurements of the teeth of beavers of different species 

 (tables II and IV) : 



(1) All the superior molars measured have the transverse diam- 

 eter equal to or exceeding the antero-posterior, except in the following 

 cases: in no. 12104, Vancouver Island (M 1 , M 2 , M 3 ) ; no. 12103, Van- 

 couver Island (M\ M 2 ) ; no. 12111, Vancouver Island (M 3 ) ; no. 12101, 

 Vancouver Island (M 3 ) ; no. 3672, Skagit River, Washington (M 3 ) ; 

 in nos. 209 and 210, Admiralty Island (M 2 , M 3 ). 



(2) All the superior premolars measured have the antero-pos- 

 terior diameters equal to or exceeding the transverse, except in the 

 following cases: nos. 12101, 12104, 12111, Vancouver Island; nos. 

 174525, 174526, New Brunswick. 



(3) Inferior molars in leucodonta have the antero-posterior diam- 

 eter greater than the transverse (except M, in nos. 12111 and 12107. 

 Vancouver Island). 



(4) All the inferior premolars measured have the antero-posterior 

 diameter greater than the transverse. 



