EVOLUTION OF MAMMALIAN MOLAR TEETH 



While these changes were taking place, the upper molars remained 

 comparatively stationary in the persistence of the simple trigon, up to the 

 close of the Cretaceous period, the main change being a depression of the 

 level of the trigon. All three cusps in some groups were depressed from 

 the high secodont to the low bunodont level. In the majority of the 

 carnivorous types we find that only the protocone was depressed and that 

 the pair of outer cusps, paracone and metacone, persisted in their high 

 primitive level ; the crown being thus prepared for the transformation 

 into the true " sectorial." But in the omnivorous and herbivorous types, 

 all three cusps are depressed and the upper molars always increased their 

 crushing area by the addition of a heel or " talon," exactly analogous to 

 that previously developed upon the lower molars.* As is well known, this 

 " hypocone " is an upgrowth from the cingulum and its typical mode of 

 development is well shown in the Primates (Figs. 128-132). While this 

 was going on the trigon was also supplementing its bunodont equipment 

 by the addition of the little intermediate cusps "protoconule" and "meta- 

 conule."t These always appeared where the " talonid " abuts against the 

 " trigon." Thus, finally, the upper molar, like the lower, was provided 

 with six cusps and both were ready to diverge into any ungulate form. 



All these foregoing stages persist and may be readily studied and 

 verified among some of the living marsupials, insectivores, lemurs and 

 monkeys, and can be seen in any well-equipped osteological museum 

 almost as well as among the fossil series. 



The Nomenclature of the Molar Cusps and Crests. 



The system proposed by the writer some years ago has now been 

 adopted by many of the American, English and German writers who are 

 studying the fossil series. It is based upon simple principles : 



1. The termination "-cone" is employed for all the primary central 

 cusps derived from the crown of the tooth, while the diminutive -conulc is 

 employed for the smaller " intermediates " or cuspules. 



2. All peripheral cusps or elements developed mainly from the cingu- 

 lum or external borders of the crown are distinguished as -styles (" pillar " 

 or " buttress "). The only exception is the " hypocoue," which, while 

 arising from the cingulum, soon takes its place upon the crown. 



* [The hypocone of the upper molars is analogous in position to the hypoconid of the 

 lower molars, but in function it is more analogous to the paraconid which it replaces, since 

 it fits into the space between the entoconid of one and the metaconid of the following lower 

 molar. A closer functional analogy is with the protocone of the upper molars, since the 

 protocone and hypocone of one upper molar fit into the talonid (hypoconid) and trigonid 

 of two successive lower molars. Eu.] 



t [In many, if not all, cases the protoconule and metaconule were developed long before 

 the hypocone. Compare the Jurassic and Cretaceous Trituberculates (pp. 96, 218, 220), 

 Pomtolambda (Fig. 140), and certain insectivores (p. 128). ED.] 



