1883,] 325 (Cope. 
posterior base of the crown. ‘Transitional states are seen in some of the 
Periptychide (Anisonchus), and in the sectorials of the Procyonide. 
The tritubercwlar 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, Pantolambda and the Amblypoda generally ; 
in Centetes and Talpa; and in its sectorial form, in Stypolophus and 
Oxyeena, ete. 
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 
of 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 quadritubereular superior molars, that the 
anterior triangle of the inferior molar is not elevated, if present, as for in- 
stance in Rhinocerus. 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,{ 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, 
PIN the origin of the sectorial tooth of the Carnivora, American Naturalist, 
S75, 
