EVIDENCE BEARING ON TOOTII-CUSP DEVELOPMENT IO3 



tive type, and on its representatives in the lower Eocene he 

 founded the tritubercular theory. That this type is primitive 

 and many, at least, of the later forms have been derived from 

 it, have been too conclusively demonstrated by Cope, Osborn, 

 Scott and others to be seriously questioned ; but this early 

 trigonodont form, as is now evident, was derived in a totally 

 different way from that assumed by the tritubercular hy- 

 pothesis. 



An especially interesting feature in these Laramie forms is 

 the oft-repeated appearance in the upper molars of a back- 

 wardly extended outer heel-like cusp connected by an elevated 

 ridge with the postero-external cusp. This portion of the tooth 

 is thus converted into a more or less perfect sectorial, or cutting, 

 blade, against which the anterior blade of the trigonid shears, 

 while the greatly broadened heel or talonid of the lower molar, 

 extending backward under the antero-posteriorly expanded 

 protocone of the upper molar, forms a successful crushing 

 apparatus. Thus, so early as the Cretaceous the prevailing 

 molar types were about equally equipped for use as cutting or 

 crushing mechanisms. The creodonts and carnivorous marsu- 

 pials seem to have early taken advantage of the sectorial blade 

 to the neglect of the crushing heel which gradully diminished 

 in relative size and importance, while in many other forms, 

 using the crushing portion of the tooth most, the sectorial blade 

 was early lost. 



Another special character marking the advance of the upper 

 Cretaceous mammal molars is the first indication in a few forms 

 of the postero-internal cusp [hypocone), which forms the fourth 

 main cusp in the later quadra-tubercular type of molars. This 

 cusp has apparently been derived, according to the evidence of 

 these Laramie types, from independent sources in different 

 groups of mammals. In a form which Marsh has referred to 

 Tclacodon a strong cone-shaped cusp has developed on the 

 postero-internal cingulum of the tooth indicating the deriva- 

 tion of the hypocone from that source. Another form, appar- 

 ently representing an undescribed genus (PI. V, fig. 7) is 

 evidently developing a hypocone from the primitive posterior 

 intermediate cusp. Still another form, represented by Proto- 



