THE TEETH 447 



stage, as we shall see, the carnivorous and sectorial dentition is 

 derived. 



(5.) The upper molar while in the tritubercular stage may develop 

 small cusps in the lines connecting the protocone with the paracone 

 and the metacone. The anterior of these cusps is termed the proto- 

 conule, the posterior the metaconule. The great change in the upper 

 molar forming the fifth stage is produced by the addition of a posterior 

 inner cusp or hypocone to the crown, with loss of the anterior inner 

 cusp or paracone. The heel of the lower molar is elevated and some- 

 times develops a median cusp or hypoconulid. The teeth thus become 

 opposite and quadritubercular (Fig. 370, G, H). From this quadri- 

 tubercular type almost all the higher mammals may be traced back 

 through successive stages of simplification. " This is the type of the 

 Quadrumana and Insectivora as well as the inferior true Ungulates. 

 The Hyracoidea and higher Ungulates add various complexities. 

 Thus the tubercles become flattened, then concave, so as to form Y's 

 in the section produced by wearing, or they are joined by cross folds 

 forming various patterns. In the Proboscidia the latter become multi- 

 plied so as to produce cross-crests." 



Evolution of the Sectorial Dentition. The inferior sectorial 

 of the cat is derived from the tuberculosectorial type by the gradual 

 loss of the heel and of the metaconid and the enlargement of the 

 paracouid and the protoconid (compare I and J, Fig. 370). The 

 number of lower molars is gradually reduced to one. " The genesis 

 of the superior sectorial tooth has been explained as follows. In con- 

 sequence of the fact that the lower canine tooth shuts anterior to the 

 superior canine, the result of the enlargement of the diameters of those 

 teeth will be to cause the crowns of the inferior teeth to be drawn 

 from behind forward against those of the superior teeth when the jaw 

 is closed (Fig. 370, I). Thus a shearing motion would result between 

 the anterior external edge of the lower triangle and the posterior 

 internal edge of the superior triangle. Now the characters of the 

 true sectorial teeth consist in the enormous extension of these same 

 edges in a fore-and-aft direction, the inferior shutting inside of the 

 superior. To account for the development of these blades. we must 

 understand that the oblique pressure of the front edge of the lower 

 tooth on the hind edge of the superior tooth has been continued for 

 a very long time. We must then observe that the internal tubercle 



