ORIGIN OF SPECIALIZED TEETH OF THE CARNIVOEA. 365 



cusps of both superior and inferior teeth engaged in this process 

 have developed in elevation, at the expense of those not engaged 

 in it, viz. : the internal cusps of the same teeth. The atrophy of 

 the latter can not have been due to friction, since the internal 

 cusps of the inferior series, which have not been subjected to it, 

 are reduced like those of the superior sectorial, which have. In- 

 deed, it is possible that some of the Creoclonta, the carnivores of 



Fig. 66. — Oxyozna lupina Cope, jaws, one half natural size, from the Wasatch beds 

 of New Mexico. Fig. a, maxillary bone with teeth, from below ; A, last superior molar, 

 from behind. Original; from the Report U. S. G. G. Survey W. of 100th Mer., G. M. 

 Wheeler. 



the Lower Eocene, may have been derived from ancestors without 

 or with rudimental inner cusps. In any case the effect of use in 

 lengthening the external cusps appears to have operated in the 

 Carnivora, as it has done to a greater degree in the Ungulata ; and 

 the lateral vertical wear would appear to have resulted in the 

 blade-form., as transverse wear in the Ungulates has resulted in 

 the plane grinding surface. 



The specialization of one tooth to the exclusion of others as a 

 sectorial, appears to be due to the following causes. It is to be 

 observed, in the first place, that when a carnivore devours a 

 carcass, it cuts off masses with its sectorials, using them as 

 shears. In so doing it brings the part to be divided to the angle 

 or canthus of the soft walls of the mouth, which is at the front of 

 the masseter muscle. At this point, the greatest amount of force 

 is gained, since the weight is thus brought immediately to the 

 power, which would not be the case were the sectorial situated 

 much in front of the masseter. On the other hand the sectorial 

 could not be situated farther back, since it would then be inac- 

 cessible to a carcass or mass too large to be taken into the mouth. 



The position of the sectorial tooth being thus shown to be de- 



