KINETOGENESIS. 333 



cipal cusps, or protocone and protoconid, alternate with 

 each other, the cusplets stand opposite to them in the 

 closing of the jaws, and a certain amount of interfer- 

 ence results. As the lesser cusps are the less resistant 

 to the wedging pressure of such contact, their position 

 would change under its influence, rather than the large 

 central cusps. The lower jaw fitting within the upper, 

 the effect of the collision between the major cusps of 

 the one jaw, and cusplets of the other, would be to 

 emphasize the relation still more ; that is, the cusplets 

 of the upper jaw would be wedged outwards, while 

 those of the lower jaw would be pressed inwards, the 

 major cusps retaining at first their original alternate 

 position. With increase of the size of the teeth the 

 cusps would soon assume in each jaw a position more 

 or less transverse to that of the other jaw, producing, 

 as a result of the crowding, a crown with*a triangular 

 section in both. The process may be rendered clear 

 by the following diagram : 



•^^^:i5^B^^^' 



Fig. g8. — Diagrammatic representations of horizontal sections of tricuspi- 

 date molars of both jaws in mutual relation ; the shaded ones represent those 

 of the upper jaw: A, Triconodon ; B, Menacodon; (7, ideal tritubercular mo- 

 lars, approached by Menacodon, B, 



It is supposed on the contrary by Rose and Kiiken- 

 thal that mammalian molars which support more than 

 one cusp have been formed by the fusion of several 

 simple reptilian cones. So far as regards the higher 

 Mammalia this hypothesis is in opposition to all the 

 facts of paleontology and is not worthy of discussion. 



