Evolution of Mammalian Molars. 1069 
It follows that it is quite as essential for the comparative anato- 
mist 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 manus 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 demon- 
strated, almost beyond a doubt, which of these tubercles is homolo- 
- gous with the single reptilian cone. The writer recently (op. cit., 
p. 242) proposed the adoption of a distinct 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 prim- 
itive position and order of development, and in most instances in 
accord with their secondary position. This nomenclature can be 
extended to the secondary cusps in the sextubercular superior, and 
quinquetubercular inferior molars. The terms now in general use 
are based, for the most part, upon the secondary or acquired posi- 
tion, and in no instance upon the homologies of the cusps in the 
upper and lower molars, or even in corresponding molars of differ- 
ent genera, thus involving much confusion. For example, the 
Antero-internal cusp of the lower molar of Mioclenus is not homol- 
ogous with the antero-internal cusp of Hyopsodus, 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 Prof. Cope’s numerous essays upon the tritubercular type in 
the Tertiary Mammalia.! 
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. 
_" Professor Cope’s essays abound with discussions and notes upon the 
origin and succession of the tritubercular type. (See collection, in ‘ Ori- 
gin of the Fittest”). He has outlined the transition from the single 
cone to the tritubercular crown ‘(p. 347); the tubercular sectorial (p. 
246) ; the quadritubercular type (p. 245 and p. 359) ; the Spalacotherium 
molars as a transition to the tritubercular (p. 259). The acquisition of 
the superior and inferior quadritubercular molar (p. 361). The predic- 
tion of the discovery of Carnivora with triconodont molars (p. 365), and 
_ Of the simple tritubercular type in both jaws (p. 362). 
