44 Wortman — Studies of Eocene Mammalia in the 



It is and lias been assumed, as one of the fundamental prin- 

 ciples of this process, that the tritubercular stage of develop- 

 ment was preceded by, and was the direct result of, the addi- 

 tion of certain new cusps to a more or less simple elongated 

 conical crown. The evidence in support of this postulate rests 

 not only upon the most excellent and weighty a priori reason- 

 ing, but is derived from the premolars of any reasonably com- 

 plete and connected series of forms whose premolars were 

 complicating and whose history can be traced over any consid- 

 erable length of time. If a single tooth is selected and traced 

 through the various stages of such a series, it will be seen that 

 certain new cusps are added in a more or less definite way. 

 The first step in this advance in structure is usually the addi- 

 tion of a cusp to the posterior border, in the form of a basal 

 talon. The next is an internal cusp which arises from the 

 cingulum, and by gradual enlargement becomes the main 

 internal cusp of the tritubercular crown. It should be noted 

 just here, however, that the exact order of appearance is by 

 no means constant, for it may so happen that the internal cusp 

 has appeared before the posterior, or the two may appear 

 simultaneously. But what is of the utmost importance to a 

 proper understanding of the subject, is to note that the tritu- 

 bercular crown of every complicated premolar thus far known 

 among the placental mammals has originated by the addition 

 of these two cusps in these situations, leaving in every case 

 the main or primitive cusp at the antero-external angle. 

 Scott has demonstrated this fact so fully and conclusively* 

 that it is bound to be accepted beyond any possibility of ques- 

 tion or dispute. He has therefore designated these cusps in 

 accordance with their order of appearance, as follows: The 

 antero-external or primitive cusp, the protocone ; the internal 

 or lingual cusp, the deuterocone, and the postero-external, the 

 tritocone. 



While this Order of addition and development of the cusps 

 has thus been shown to be true of the premolars, it is held by 

 Osborn to have been otherwise in the molars. For the sake of 

 rendering his position perfectly clear, I quote at some length 

 from his otherwise excellent paper on the " Structure and 

 Classification of the Mesozoic Mammalia."f He says: "If, 

 as now seems probable, the derivation of the mammalian 

 molar from the single reptilian cone can be demonstrated by 

 the comparison of a series of transitional stages between the 

 single cone and the three-cone type, and from the latter to the 

 central tritubercular type, the separate history of each cone 

 can certainly be traced throughout the series in its various 

 degrees of modification, development, and degeneration. The 



* Proc. Acad. Nat. Sci. Phila., 1892, p. 405. 

 f Jour. Acad. Nat. Sci. Phila,, 1886, p. 242. 



