68 EVOLUTION OF MAMMALIAN MOLAR TEETH 



a crushing or grinding function, and therefore plays a chief role in all 

 ' bunodont ' types. The first diagram exhibits the relations of these two 

 portions of the crown in the upper and lower molars, and the six primary 

 and secondary cusps which typically develop upon each (Fig. 41).* 



We will not enter here into the well-understood transformation of 

 this tuberculo-sectorial type into the sexitubercular bunodont type, seen 

 typically in the upper and lower molars of the Puerco Protogonia 

 [JBuprotogonia, Fig. 149, p. 169], which is the least specialized ancestral 

 bunodont form that has been discovered. We may lay emphasis upon 

 the fact that the parent form of ungulate molar has six tubercles both 

 above and below instead of six above and four below as formerly 

 supposed. 



It is important to remember, as an exception to the law of sexitu- 

 bercular origin, that all the Amblypoda and all the Periptychidaet 1 (among 

 the Condylarthra) developed their upper molars upon the trigonal basis, 

 out of the three tubercles of the tritubercular crown, and without 

 becoming sexitubercular, that is, without the addition of the hypocone 

 or talon. 



Now how shall we study the molar teeth of the early Ungulates, 

 especially of the apparently similar primitive forms of Perissodactyls, 

 which are so difficult to distinguish ? The following steps must be 

 taken : 



First. Locate each of the six primary and secondary cusps, as far as 

 they are present. 



Second. Note the form of each, whether rounded (bunoid), crested 

 (lophoid), or crescentic (selenoid). 



Third. Note the position of each upon the crown with relation to 

 the other cusps. 



Fourth. Note the relative size or development of each. 



Fifth. Note the relative development of the cinyidum, in different 

 parts of the contour. 



Sixth. Note the presence of one or more peripheral secondary cusps, 

 which develop from the cingulum, or external borders of the crown. 



Finally. If crests are formed or forming, note the points at which 

 the transverse crests unite with the external cusps (paracone and metacone, 

 paras tyle and mesostyle). 



"[From the evidence furnished by the molars of the Jurassic and Cretaceous mammals, 

 of the Eocene trituberculate Creodonts, Insectivores, and Primates (e.g. Anaptomorphiis), it 

 seems probable that the talonid or heel in the lower molars appeared much earlier than the 

 so-called "talon" or hypocone in the upper molars. See Gidley's views on page 221, Fig. 208. 

 The hypocone of the upper molars (p. 59; Figs. 144, 147, 252), was developed part passu 

 with the degeneration of the paraconid, and fits in between the entoeonid of one lower molar 

 and the metaconid of the next lower molar. It is really a secondary crusher, analogous with 

 the protocone. ED.] 



1 There are considerable grounds for removing the Periptychidse from the Condylarthra 

 to Amblypoda. O. f [See p. 164.] 



