OBJECTIONS AND DIFFICULTIES AND OTHER THKniMKS 



described. They are composed of three nearly equal cone-like cusp- 

 arranged like those in the upper molars of this genus in an antero- 

 posterior line. There is no cusp corresponding with the metacuni'l 

 in Dryolestes. There is a continuous basal cingulum on the inner face 

 of the crown, and the posterior cusp is in no way homologous, except 

 in position, to the heel in the lower molars of Piun-mlon and 

 Dryolestes. 



' The mammals from the upper Cretaceous Laramie beds show a 

 great advance in development. The molars of the trituberculate forms 

 of this horizon have passed into a second well-defined stage of special- 

 ization which, though varying greatly in detail in the various types, 

 conforms in general to a distinctive pattern which may readily have 

 been derived from some Atlantosaurus-beds form, such as Dryolestes. 

 An upper molar of Pcdiomi/s Marsh, a typical example of the Laramie 

 tritubercular molar, compared with the corresponding tooth of Dryolestes, 

 presents the following differences and indicates the principal lines of 

 progression : 



"(1) The main internal cusp (protoconc) is much broadened 

 antero-posteriorly ; (2) a second small V-shaped intermediate cusp 

 ( protocomde) has been added; (3) the postero-external cusp (metacone) 

 has greatly increased, nearly equaling, both in size and importance, the 

 median external, or primary, cone (paracone), while the antero-external 

 cusp (parastyle) has remained small and undeveloped. A correspond- 

 ingly progressive development marks the trigonid and heel of the lower 

 molars." 



Summary ami Conclusions [G 



" Summing up the evidence derived from this preliminary study, the 

 following conclusions are suggested : 



' 1. That the evidence obtained from the Mesozoic mammal teeth 

 furnishes no support to the tritubercular theory in so far as it involves 

 the position of the protocone and the derivation of the trigonodont tooth 

 from the triconodont stage through the shifting of the lateral cones 

 outward in the upper molars and inward in the lower molars. 



" '2. That it supports entirely the embryological evidence that the 

 primary cone is the main antero-external cusp, or paraconc, having 

 retained its position on the o/'/s/W/ in most upper molars (see exceptions 

 above, p. 124). 



"3. That it agrees in the main with Huxley's ' premolar analogy 

 theory, as supported by Scott. 



" 4. That the molars of the Multituberculates, Triconod'on, Dryolestes, 

 and Dicrocynodon, were apparently derived independently from the simple 

 reptilian cone : hence the supposition follows that the trituberculate type 



