42 EVOLUTION OF MAMMALIAN MOLAR TEETH 



This pattern is repeated, with a considerable elevation of the heel, 

 in Peramus (Fig. 18) of the upper Jurassic. 1 Neither of the two 

 foregoing are of the primitive heelless tritubercular type which is 

 apparently found in Spalaeotherium (Figs. 11), also upper Jurassic, 

 and in the nearly related if not synonymous Peralestcs (Fig. 12). 

 Contemporary with the above, are numerous genera of the Stylodon 

 order; among these, Astlteiioihn (Fig. 35) is of the primitive tri- 

 tubercular 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 bunodont forms are known they were probably developed during the 

 Cretaceous, for a few are found well developed in the Puerco. In the 

 Secodont 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 nieta-conules in the 

 upper molars and the appearance of the cntocouid upon the inner side of 

 the hypoconid below. 



The Principles govern in// Cusp Development. It is remarkable to note 

 in how many particulars the actual succession of molar development in 

 the Mesozoic period coincides with the theoretical scheme of origin of 

 trituberculy proposed by Cope 2 and supported by Wortman 3 several 

 years ago. At that time Spalaeotherium and the genera now embraced 

 under the Triconodontidee 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 

 tritubercular 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 complicated as to require a separate treatment. The 

 balance of evidence in tritubercular evolution seems to favor the second 

 or kinetogenesis theory as apparently witnessed in two laws of cusp 

 development : 



I. The primary cusps first appear as cuspules, or minute cones, at the 



1 This genus includes also Leptocladus (Inhiiix Owen, and Spalaeotherium minus 

 Oweii (Fig. 18). See Proc. Phila. Acad., Nov., 1888, p. 292. 



2 " The Evolution of the Vertebrata Progressive and Retrogressive," American 



t, April, 1885, p. 350. 



3 "The Comparative Anatomy of the Teeth of the Vertebrata," 1886, p. 418. 



