TEETH OF THE MESOZOIC MAMMALS 35 



upon its slopes.* The superior molars of Peralestes,^ however, when 

 viewed from above, present one large internal and two smaller external 

 cusps disposed in a triangle opening outwards, and as this is the general 

 disposition of superior cusps of the tritubercular type, we must admit 

 the possibility that the smaller cusps do represent the para- and nieta- 

 cones in a stage of inward rotation not accompanied by the production 

 of the sectorial blades, for this is by no means an essential feature 

 of the tritubercular molar. The history of the derivation of the 

 molars of the Peralestida; must, therefore, be left in some doubt ; 

 while the balance of evidence points to a line of development similar 

 ti that in progress in the Amphitheriidse, although the line of descent 

 appears to be different. 



Eeviewirig this study of the molars the following are the principal 

 deductions : (a) The molars of all the mesozoic mammals of this 

 group present one main cusp which is either so central or so pro- 

 minent that it may be considered homologous with the single reptilian 

 cone or protocone. (6) In one line of genera two lateral cusps, the 

 para- and meta-cones, appear upon the anterior and posterior slopes 

 of the protocone. This is a central and frequently repeated stage 

 of evolution. It gives rise to two lines of molar development ; in the 

 first, the para- and meta-cones are retained in the same fore and 

 aft line, as the persistent triconodont type, but increase greatly in 

 size ; in the second, they are rotated inwards as the tritubercular 

 type, which finally acquires a heel, (c) In a second line of genera 

 the paracone appears upon the anterior slope of the protocone, but 

 the metacone is not developed, being replaced by a basal talon or 

 hypocone which extends inwards to form the internal cusp. (V) In 

 a fourth line of genera neither the para- nor meta-cones are developed 

 upon the sides of the protocone, but they are replaced by basal 

 cusps derived from the cingulum. 



* [Per contra the conditions seen in Amphitherium may have been derived from those 

 seen in PeraspaJax. ED.] 



t [Supposed to correspond with the lower molars of the Spdlacotherium type, Fig. 11. 

 ED.] 



