SKCOX1) OUTLINE OF TRITUIJKRCULY 83 



3. The crests, transverse and longitudinal, are always composed of 

 two or more cusps and styles, and are distinguished by the termination 

 -lopli. 



4. The prefixes "proto-" "para-" ' meta-," "hypo-" " ento-," etc., 

 refer back to the primitive position or order of development in the tricon- 

 odont and tritubercular stages. 



5. The suffix -id is employed arbitrarily to distinguish the elements of 

 the lower molars from those of the upper. 



The use of the terms " trigon " and " talon " for the cutting and crush- 

 ing regions of the crown, respectively, is especially advantageous among 

 the upper Mesozoic and lower Cainozoic mammals, where it is necessary 

 to refer constantly to the relations of the upper and lower crowns in 

 apposition, as in the evolution of the sectorial and lophodout types. As 

 to the form of the cusps, we pass from simple pointed cusps to three well- 

 known modes of modification to which the adjective " bunoid," " lophoid," 

 and " selenoid " may be applied. A combination of these terms gives us a 

 permanent system of distinguishing the complex forms of ungulate molars 

 from each other, by referring first to the form of the protocone ; second, to 

 that of the outer paracone and metacone. Thus in Palceosi/ops, as the 

 protocone is bunoid and the outer cusps are selenoid, the crown may be 

 distinguished as " buno-selenodont." In Pcdceotherium the protocone is 

 "lophoid," and it may be described as " lopho-selenodont." Rhinoceros is 

 truly " lophodont," since all its six cusps are " lophoid." These are pre- 

 ferable to the terms " tapirodont," " symborodont," " bathmodont," "loxolo- 

 phodont," etc., proposed by Cope, because the latter are associated with 

 generic types. 



The Evolution of the Ungulate Molar. 



The fact of derivation of all ungulate molars (excepting in the Ambly- 

 poda) from sexitubercular upper and lower crowns, leads us to look sharply 

 for traces of these six tubercles [as modified] from the primitive plan of 

 Euprotogonia. These six cusps are almost invariably found in the upper 

 molars of both perissodactyls and artiodactyls up to the middle of the 

 Eocene period, as typified in Hyracotherium and Homacodon or Dicholune. 

 In the lower molar the trigon loses the " paraconid " and the talon loses 

 the " hypoconulid," the latter persisting only in the last molar as the 

 " third lobe." This loss was accompanied by the complete transformation 

 of the lower molars from the " secodont " to the comparative " bunodont " 

 type, as effected in the lowering of the " trigonid " to the level of the 

 " talonid." This is exemplified in the steps between the first and third 

 molars of the creodont genus Miacis (Fig. 38, No. 6). In a side view of 

 all early ungulate molars, such as Hyracotherium, we see that the " trigonid " 

 is still the highest portion of the crown. In the ungulates, unlike the 



