34 MICROSCOPIC ANATOMY OF THE TEETH 



lower molars, disposed in triangles '. He concluded that 

 * this tritubercular type was ancestral to many if not all 

 of the higher types of molar teeth ', and considered that it 

 arose from a single conical type or reptilian form of tooth 

 by the addition of added denticles. 



It is seen that the concrescence theory also derives the 

 molar teeth from a single reptilian cone, but in a different 

 manner, the supporters of this theory considering that by 

 a shortening of the jaw the multiple conical teeth of the 

 reptiles underwent a clustering or concrescence, forming 

 multicuspidate teeth, and that tritubercular teeth were 

 not the earliest type derived from the single reptilian cone. 

 They thus consider that a molar tooth with numerous 

 tubercles was the more primitive form. 



According to the tritubercular theory, the primitive form 

 from which the mammalian molar arose was a single cusp, 

 and additional cusps have been successively added to this. 

 The tooth with a single conical crown has not hitherto 

 been found in any known ancestor of the Mammalia, but 

 is described as the haplodont type (Plate II, fig. AJ. 



A single main cone, however, furnished with two small 

 accessory cusps, occurs in the fossil Dromotherium (fig. A 2 ), 

 and is described as the protodont type. A central cone with 

 two well-developed accessory cusps gives the triconodont 

 type. 



Cusps of This form is represented in the extinct Triconodon (fig. A 3 ), 



bercidar anc ^ ^ rorQ this we P ass * ^ ne tritubercular type (fig. AJ, 



tooth. in which there is a triangular crown with three main cusps, 



called respectively the protocone, paracone, and metacone, 



the protocone being internal in upper molars, external in 



lower molars. The corresponding cusps in the lower jaw 



are distinguished by the affix -id, being called protoconid, 



paraconid, and metaconid respectively. 



Trigon According to Cope and Osborn (18) the molar is made up 

 on ' of the primitive triangle or trigon and the added ' heel ' or 

 ta'on (fig. A-). 



The trigon or primitive triangle is the sectorial or tearing 

 element of the mammalian tooth ; the talon, the crushing 

 or masticating element. 



Upper Molars. According to Osborn, in the early lemurs 



