1907] On a Tooth of Ovibos 17 



this lake would have taken place when the tooth had been worn 

 down about 12 mm. more. The transverse section (Fig. 1 <;) a 

 little below the mid-height of the tooth (at d, Fig. 1) shows the 

 posterior lake isolated with the addition near the inner division 

 point of the lobes of the "small accessory valley (e, Fig. 1 c), to 

 which attention is called by Dr. E. Lonnberg in his paper 'On 

 the Structure and Anatomy of the Musk-ox."* 



In the Fort Rae musk-ox the first and second upper true 

 molars show this accessory valley well developed, and the third 

 molar, which is not so much worn as the other two teeth, shows 

 it in process of formation, but still attached to and continuous 

 with the anterior cement lake. In this specimen only the small 

 portion of the teeth above the alveolar border is available for 

 examination. 



In the specimen of Ovis montana neither of the cement lakes 

 in the grinding surface of the last upper molar (very little worn) 

 are completely enclosed; the anterior one communicates with 

 the inner longitudinal furrow and also by a narrow surface with 

 the posterior lake. With further wear (Fig. 2, section at mid- 

 height of tooth) the two lakes become enclosed and distinct, but 

 without the formation of the "small accessory valley." A second 

 section nearer the base of the tooth reveals this small valley 

 well formed. The first upper true molars in the same skull show 

 this valley very plainly in the grinding surface, and it appears 

 in a section at mid-height in the second molar. The "small 

 accessory valley" is thus seen to be developed in both the musk- 

 ox and the mountain sheep in the true molars. The styles of 

 the Rock Creek tooth have about the same prominence and 

 thickness as those of the sheep. 



The Rock Creek tooth is without the "accessor}'- column" 

 that is stated to arise in Ovibos* at the base of the inner surface 

 of the molars between the two lobes. This column is, however, 

 apparently absent in the third upper molarf of Ovibos. In the 

 second and third upper molars of the mountain sheep examined 

 there is no trace of this column. 



Measurements of the Rock Creek tooth (moderately worn), 

 and those of the corresponding tooth in Ovibos moschatus 



Proceedings of the Zoological Society of London for the year 1900, 

 P-712. ^ veil 



Lonnberg, op. cit., p. 712. ; vV)"'^-''^^ 



tOsgood, op. cit., p. 17 7. /^i^yJ^O^^^i^y^s. O^ 





