1078 Evolution of Mammalian Molars. 
treated by Riitimeyer, Kowalevsky, Cope, Schlosser and others. 
The Relation of Trituberculy to the Persistence of Mammalian 
Phyla.—The above table shows somewhat indefinitely, but none the 
less positively, the general progression of the Mammalia, to and 
from the primitive tritubercular type. As already stated, even with 
our present very limited knowledge, certain stages appear to have 
been characteristic of certain periods, as follows: the triconodont 
in the lower Jurassic; the primitive tritubercular and tubercular 
sectorial in the upper Jurassic; the secodont and bunodont sub- 
types of trituberculy, predominated in the Puerco ; in the Bridger, 
the Perissodactyl ungulates had mostly passed beyond into the 
lophodont and symborodont types, and the Artiodactyls were 
approximately in the stage of sub-types c; but the Lemuroidea, 
Creodonta, Insectivora, etc., were, almost without exception, tritu- 
bercular. | 
There can be little doubt that, parallel with the tritubercular 
forms, in each period, there were aberrant or degenerate types, but 
it is difficult to determine which these are. Many Mesozoic types, 
which the writer formerly considered aberrant, have now proven to 
be tritubercular The upper Jurassic genera included under the 
Dicrocynodontide (see Marsh, Amer. Journ. Se., April, 1887, p. 
338) are apparently aberrant. There are several degenerate types 
among the Puerco and Wasatch Creodonts, such as Dissacus and 
Mesonyx. But there is a striking proof of the superiority of the 
tritubercular molar in the fact that, according to our present know- 
ledge at least, the Jurassic mammals possessing aberrant or degen- 
erate molar types did not persist into the Puerco, nor did such 
types in the Puerco persist into the Bridger. There is some doubt 
as to the persistence of the sub-tritubercular stage; the writer pre- 
viously considered the Thylacinus molars as triconodont ; but Mr. 
Lydekker has called attention to the probability that the metaconid 
has disappeared and been replaced by a heel as in the sectorial teeth 
of the Carnivora. The disappearance of the degenerate types ney 
be attributed to the general principle that rapid specialization and 
loss of parts leads ultimately to extinction, by depriving the anima 
of the means of adaptation to new conditions, or surroundings. 
* See ‘‘ Additional Observations upon the Structure and Classification 
_ Of the Mesozoic Mammalia.” Proc. Phila. Acad., Nov., 1888. see : 
