102 GIDLEY 



From these observations it seems apparent that the trigonid 

 of the lower molars is not the reverse of the trigon of the upper 

 molars, as held by advocates of the tritubercular theory, and 

 the homologues of the elements of the upper and lower molars, 

 as proposed by this theory, are far from being apparent. (This 

 also accords with the conclusions of Winge.) 



The lower molars of Triconodon differ from any of the forms 

 just described. They are composed of three nearly equal 

 cone-like cusps arranged like those in the upper molars of this 

 genus in an antero-posterior line. There is no cusp corres- 

 ponding with the metaconid in Dryolestes. There is a continu- 

 ous 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 Paurodon and Dryolestes. 



The mammals from the upper Cretaceous Laramie beds show 

 a great advance in development. The molars of the tritubercu- 

 late forms of this horizon have passed into a second well-defined 

 stage of specialization 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 Pediomys 

 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 : 



(i) The main internal cusp (prolocone) is much broadened 

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

 cusp (protoconule) 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 re- 

 mained small and undeveloped. A correspondingly pro- 

 gressive development marks the trigonid and heel of the 

 lower molars. 



Thus, the " trigonodont " tooth, or a type of molar with three 

 principal cusps of almost equal importance, arranged in the 

 form of a triangle, makes its first appearance in the Laramie. 

 This pattern of tooth Cope early recognized as a general primi- 



