84 EVOLUTION OF MAMMALIAN MOLAR TEETH 



carnivores, all three molars were affected simultaneously. An exactly 

 similar levelling process can now be observed in a comparative series of 

 recent Lemurs and Monkeys. To summarize the five steps toward the 

 establishment of the ungulate primitive type : the addition of the lower 

 talonid, the lowering of the cusps of the upper trigon, the addition of the 

 upper talon and simultaneous lowering of the lower trigonid, the loss of 

 the paraconid and hypoconulid. By these changes the cutting was trans- 

 formed into the crushing type. The development of the talon necessitated 

 the loss of the " paraconid," for they both occupy the same space when 

 the jaws are closed ; the stages of this gain to the upper molar and loss 

 to the lower are well shown in the species of JZuprotogonia. 



All these changes belonged to the constructive period and took place 

 presumably before the great divergence of the ungulate orders began ; or 

 it may have been partly due to parallelism or homoplasy, because we find 

 that the molars of Triyonolestes, the earliest known artiodactyl, are tritu- 

 bercular.* Some groups, such as those to which Coryphodon, Uintatherium 

 and Periptyclms belong, built up their whole molar structure upon the tri- 

 tubercular or trigonal basis. 



From this point onward dated the period of " modernization." An 

 important legacy of the old triangular form was the obliqur arrniit/emnit of 

 the outer and inner cusps parallel with the sides of the primitive triangles. 

 Thus all the primitive crests developed upon these cusps were oblique and 

 not directly transverse. The main features of modernization upon which 

 we must now closely direct attention are : 



1. The addition of one or more peripheral cusps or "styles" as up- 

 growths from the cingulum. These reached their most extreme develop- 

 ment in the EquicUe. (See Fig. 49.) 



2. The persistence or degeneration of the cingulum at certain points, 

 for all primitive molars are completely invested by a broad cingulum.''" 



3. The modelling of the cusps into the " bunoid," " lophoid " or 

 " selenoid " form. 



4. The metatrophic or unequal growth of the cusps, especially as 

 affecting the external pair, protocone and metacone, in the upper molars. 



5. The shifting of the cusps from their primitive position upon the 

 crowns. 



6. The shifting point of union of these transverse crests with the 

 external crest. 



The differential features of the development of ungulate molars all 

 group around these six heads. If we were examining an isolated molar 

 tooth from the lower Eocene, the first step would be to locate its primary 

 cusps and then note its divergence as tested by the above differentia. 

 We would then be in a position to make a conjecture as to the series in 



* [See pages 171, 172. ED.] t [More or less. ED.] 



