1074 Evolution of Mammalian Molars. 
the primitive tritubercular type without the hypoconid, all the 
remainder present various modifications of the tubercular sectorial- 
This covers our knowledge of trituberculy in the Mesozoic 
period. No bnunodont forms are known—they were probably 
developed during the Cretaceous, for a few are found well 
developed in the Puerco. In the Sectorial series many of the 
types do not widely depart from those seen in the Jurassic, but 
the Bunodont series are universally characterized by the initial 
or advanced development of the proto- and metaconules in the 
upper molars and the apperance of the Entoconid upon the inner 
side of the hypoconid below. 
The principles governing cusp development.—It is remarkable to 
note in how many particulars the actual succession of molar devel- 
opment in the Mesozoic period coincides with the theoretical 
scheme of origin of trituberculy proposed by Cope! and supported 
by Wortman ? several years ago. At that time Spalacotherium and 
-the genera now embraced under the Triconodontide were the only 
Mesozoic mammals whose molar structure was fully known, and 
the views of these authors were partly speculative and partly 
deductive from recent dental anatomy. 
Two hypotheses may be advanced to explain the evolution of the 
ritubercular type. The first is that the type has been acquired by 
the selection of accidental variations in the production of new cusps ” 
and modelling of old ones. The second is, that the interaction of 
the upper and lower molars in the movements of the jaws has 
resulted in local increase of growth at certain points, resulting first 
in new cusps, then in a change of position and of form in the 
_cusps. Both hypotheses are open to numerous objections and are 
by no means mutually exclusive, but the whole subject is so com- 
plicated as to require a separate treatment. The balance of evi- 
dence in tritubercular evolution seems to favor the second oF _ 
kinetogenesis theory—as apparently witnessed in two laws of cusp 
development. 
I. The primary cusps first appear as cuspules, or minute wee: 
1“ The Evolution of the Vertebrata Progressive and Retrogressive,’ 
American Naturalist, April, 1885, p. 350. n 1886 
ae The Comparative Anatomy of the Teeth of the Vertebrata, 
p. 418, 
