1883.] o25 [Cope. 



posterior base of the crown. Transitional states are seen in some of the 

 Periptychidae (Anisonchus) , and in the sectorials of the Procyonidse. 



The tritubercular or triangular superior molar is associated with a corre- 

 sponding form of the anterior part of the inferior molar, This kind of in- 

 ferior molar* I have called the tubercular sectorial, and is very variable 

 as to the degree of development of the sectorial cutting edge. The anterior 

 triangle is formed by the connection by angle or crest, of the median and 

 anterior internal crests with the anterior external. Its primitive form is 

 seen in Didelphys, Pelycodus,Pantolambcla and the Amblypoda generally ; 

 in Centetes and Talpa; and in its sectorial form, in Stypolophus and 

 Oxysena, etc. 



The mechanical action of such teeth is as follows : Of course, it results 

 from the form of the superior molars that the spaces between them are 

 wedge-shaped, the apex external, the base opening to the palate. The base 

 ot the triangular section of the anterior part of the inferior molar is inte- 

 rior, 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 inferior 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 dentition of most mammalia. In so far it re- 

 sembles 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 posterior internal lobe, which produces 

 the quadrituberculate type. This lobe, by opposing the anterior internal 

 lobe of the next posterior inferior molar, precludes the entrance of the an- 

 terior triangle of the latter between the two superior molars. Hence we 

 find in the types which possess quadritubercular superior molars, that the 

 anterior triangle of the inferior molar is not elevated, if present, as for in- 

 stance in Rhinoceru3. It is, however, more frequently atrophied, and dis- 

 appears, forming the inferior quadritubercular molar so well known. 



On the other hand, as I have pointed out,f 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 modi- 

 fication, 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 correspond- 

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

 the sectorial tooth. 



* See Report G. M. Wheeler, D. Chief of Engineers on Explor. Surv. "W. 100th 

 Mer. Vol. IV, pt. ii ; on the Creodonta. 



t On the origin of the sectorial tooth of the Carnivora, American Naturalist, 

 1875, 



