1!»1().] 



Foundations of the "Wedge Theortj." 



185 



Mechanics of molar evolution in the Jurassic. 



The molars of Peralestes (Fig. 9, p. 174) seem not to have been derived 

 from the Spalacotherium or Triconodont type with symmetrical trigon. 

 The upper molars of the remote ancestors of the Trituberculata as repre- 

 sented by both Drijolestes and Peralestes may on the contrary be conceived 

 as follows (Fig. 11): The molars were small and many in number (more 

 than six). The transverse diameter of each 

 crown was relatively great, the crown was 

 much narrower on the inner side, and hence 

 roughly cuneiform in cross section; the 

 narrower internal side culminated in a high 

 rounded tip, the protocone. Each tooth was 

 supported on two large roots, an internal 

 and an external one. The teeth were well 

 spaced. 



The lower molars alternated with the 

 upper but were much smaller, they were 

 also slightly compressed in cross section at 

 the base and were supported on a com- 

 pressed root which had already become 

 constricted in the middle. Each roughly 

 pyramidal protoconid was sub-triangular in 

 cross section, the rounded apex of the 

 triangle being external. When the jaws 

 engaged with each other the protoconid of 

 each lower molar fitted into the space 

 immediately in front of the protocone of the 

 corresponding upper molar. The tip of the 

 protoconid was slightly external (buccal) to 

 the tip of the protocone (Fig. 11) but 

 internal (lingual) to the broad external base 

 of the protocone, the site of the future para- 

 style and external cingulum. 



Here are all the essential conditions for 

 a future tritubercular dentition, without 



any migration of cusps, and with the upper and lower main cusps located 

 in exactly the same positions with respect to the inner and outer sides of the 

 tooth and to each other which they occupy in typical tritubercular types. 



The talonid on the lower molar was initiated through the circumstance 

 that each lower molar, being pulled from behind, u})ward and forward, 



me 



A 



tat 



en- 



Fig. 11. Hypothetical recon- 

 struction of the upper and lower 

 molars in the ancestors of the Tri- 

 tuberculata. 



A. Internal view showing para- 

 conid (pa.d) metaconid, (me.d) ento- 

 conid (en.rf) growing up from the 

 internal cingulum; protoconid (pr.d) 

 pyramidal, talonid (tal.d) receiving 

 tip of large protocone ipr.) of upper 

 molars. Right side. 



B. Projection of upper on lower 

 molars. Protoconids and metal- 

 ophids fitting between protocones; 

 paracone (pa) centro-external, inci- 

 pient. 



