TRITUBERCULATE TYPE OF MOLAR TOOTH. 361 



ing to the palate. The base of the triangular section of the an- 

 terior part of the inferior molar is interior, and the apex exterior, 

 and when the jaws are closed this triangular prism exactly fits 

 the space between the superior molars. The lower heel of the in- 

 ferior molar receives the impact of the crown of the superior 

 molar. Thus the oblique edges of the inferior triangle shear on 

 the edges of two adjacent superior molars. The anterior parts of 

 the inferior molars, and the superior molars, form an alternate 

 dental series as distinguished from the prevalent opposed denti- 

 tion of most Mammalia. In so far it resembles the reptilian 

 dentition. 



This primitive dentition has been modified in two directions, 

 viz., to form the grinding and the sectorial dentitions. As 

 already remarked, the superior molars gradually acquire a pos- 

 terior internal lobe, which produces the quadrituberculate type. 

 This, lobe, by opposing the anterior internal lobe of the next pos- 

 terior inferior molar, precludes the entrance of the anterior trian- 

 gle of the latter between the two superior molars. Hence we find 

 in the types which possess quadri tubercular superior molars, that 

 the anterior triangle of the inferior molar is not elevated, if pres- 

 ent, as, for instance, in Rhinocerus. It is, however, more fre- 

 quently atrophied, and disappears, contributing to form the in- 

 ferior quadritubercular molar so well known. 



On the other hand, as I have pointed out,* the anterior 

 internal cusp of the triangle of the inferior molar may be more 

 developed antero-posteriorly, giving the antero-internal edge of 

 the triangle much greater obliquity than the postero-internal. 

 In correspondence with this modification, the superior triangular 

 molar loses its equilateral character by the more anterior position 

 of its internal angle, thus elongating the posterior internal side of 

 the crown. The latter thus fits the corresponding form of the 

 triangle of the inferior molar, forming with it the shear of the 

 sectorial tooth. 



In a former article, ^^On the Homologies of the Molar Teeth," 

 etc., I traced the modifications of the superior and many of the 

 inferior molars of the ungulate mammals to a parent quadrituber- 

 culate type. In a subsequent essay f I traced the origin of the 



* On the origin of the sectorial tooth of the Carnivora, " American Naturalist," 

 1875. 



f "Journal Academy Natural Sciences," Philadelphia, March, 18Y4. 



