38 EVOLUTION OF MAMMALIAN MOLAR TEETH 



existing types of molars were well advanced, but trituberculy persisted 

 in the dentition of several orders, in which it is found to-day (Lemu- 

 roidea, Insectivora, Carnivora, and many Marsupialia). 



It follows that it is quite as essential for the comparative anatomist to 

 thoroughly grasp the meaning and history of each of the component cusps 

 of the tritubercular molar and of their derivatives, as it is to perfectly 

 understand the elements of the maims and pes. For, the homologies of 

 the cusps can now be determined almost as certainly as those of the 

 digits. Take a human molar, for example, every component tubercle 

 has its pedigree, and it can be demonstrated, almost beyond a doubt, 

 which of these tubercles is homologous with the single reptilian cone. 

 The writer recently (op. cit., p. 242) proposed the adoption of a dis- 

 tinct nomenclature for the different cusps of the tritubercular molar, 

 and offered a series of terms for the primary cusps based as far as 

 possible upon the primitive position and order of development, and in 

 most instances in accord with their secondary position. This nomencla- 

 ture can be extended to the secondary cusps in the sexitubercular 

 superior and quinquetubercular inferior molars. The terms now in 

 general use are based, for the most part, upon the secondary or 

 acquired position, and in no instance upon the homologies of the cusps 

 in the upper and lower molars, or even in corresponding molars of 

 different genera, thus involving much confusion. For example, the 

 antero-internal cusp of the lower molar of Mioclcenus is not homo- 

 logous with the antero-internal cusp of Hyopsoclus, nor with the antero- 

 internal cusps of the upper molar of either genus. 



The present contribution is based principally upon the writer's 

 studies among the Mesozoic Mammalia, and, with some additions, upon 

 Professor Cope's numerous essays upon the tritubercular type in the 

 Tertiary Mammalia. 1 



Four propositions may be laid down for discussion : 



(1) That trituberculy was acquired during the Mesozoic period, in 

 a series of stages beginning with the single cone and attaining to the 

 primitive sectorial type in the Jurassic period. 



(2) The majority of Mesozoic mammals showed trituberculy in 

 some of its stages. Present evidence goes to show that the remaining, 

 or aberrant types, if such existed, did not persist. The majority of 

 the persisting forms of later periods were derived from the forms with 



1 Professor Cope's essays abound with discussions and notes upon the origin and 

 succession of the tritubercular type. (See collection in Origin of the Fittest.) He 

 has outlined the transition from the single cone to the tritubercular crown (p. 347) ; 

 tubercular sectorial (p. 246) ; the quadritubercular type (p. 245 and p. 359) ; the Spala- 

 cotherhim molars as a transition to the f>-ifnl>< rcular (p. 259). The acquisition of the 

 superior and inferior quadritubercular molar (p. 361). The prediction of the discovery 

 of Carnivora with triconodont molars (p. 365), and of the simple tritubercular type in 

 both jaws (p. 362). 



