Evolution of Mammalian Molars. 1075 
at the first peints of contact between the upper and lower molars 
in the vertical motions of the jaws. 
II. The modelling of the cusps into new forms, and the acqui- 
sition of secondary position, is a concomitant of interference in the 
horizontal motions of the jaws. 
The second law applies especially to the evolution of the molars 
after the acquisition of the tritubercular stage, and has been ably 
proposed and supported by Ryder,! principally in its application to 
recent types of teeth. The first, although not heretofore distinctly 
formulated, is partly founded upon facts and principles advanced 
by Cope, and applies chiefly to the stages which have been discussed 
in this essay. 
During the Homodont mammalian or sub-mammalian molar 
Stage, the jaws were probably isognathous and the simple cones 
alternated as in the Delphinide (fig. 1). The first additions to the 
protocone appeared upon its anterior and posterior surfaces. The 
growth of the para- and metaconids involved anisognathism,” for 
we find in the later triconodonts that the lower molars closed inside 
of the upper (Zriconodon, fig. 2). There are several transition 
forms such as Tinodon and Menacodon between the primitive tri- 
conodont type and Spalacotherium, and it has been assumed by Cope 
and the writer (op. cit., p. 243) that the para- and metaconids were 
first formed upon the anterior and posterior slopes of the protoconid 
and then rotated inwards, but it is also possible that they were 
originally formed upon the inner slopes. In the complemental 
formation of the upper and lower triangles the jaws remained 
nearly isognathous (fig. 4). There is no evidence as to the origin of 
the hypoconid, which as a rule preceded the hypocone, as it was 
developed very early. In the Stylacodontide, Phascolestes, Amblo- 
wi On the Mechanical Genesis of Tooth Forms.” Proc. Phila. Acad., 
PE ea by Ryder (op. cit., p. 45). “So as not only to indicate 
respectively parity and disparity in transverse diameter of the crowns 
of the upper and lower molars, but sad re parity or disparity in width 
transversely, from outside to outside,” 
It is clear that in the homodont bone oe with the teeth simply 
piercing the food, the greatest comminution (of the food) is effected by 
isognathism ; in the triconodont stage, the jaws must be anisognathous 
to close upon each other, but the tritubercular stage admits a return to 
isognathism by the alternation of the triangles. 
