TIIITUBERCULAR EVOLUTION IN MAMMALS 43 



first points 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 acquisition 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, 1 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 chielly 

 to the stages which have been discussed in this essay. 



GO? 



PIG. 36. Molars of opposite jaws in normal mutual relation. 1. Dclphinug. 2. Vromathcrium. 

 3. Triconotlon. 4. Spalacotkeriuin (lower), Pcralcstts (upper). 5. Vii;-,-,-tt cv.s. 6. Miodacnus. 



l V.s. 



During the homodont mammalian or sub-mammalian molar stage, the 

 jaws were probably isognathous* and the simple cones alternated as in the 

 Delphinidse (Fig. 36, No. 1). The first additions to the protocone appeared 

 upon its anterior and posterior surfaces. The growth of the para- and 

 meta-conids involved anisognathism, 2 for we find in the later triconodonts 

 that the lower molars closed inside of the upper (Triconodon, Fig. 36, 

 No. 3). There are several transition forms, such as Tinodon (Fig. 10) 

 and Menacodon (Fig. 9) between the primitive triconodont type and 

 Spalacoth&rium (Fig. 11), and it has been assumed by Cope and the writer 

 (<>p. cit., p. 243) that the para- and meta-conids 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 



1 "On the Mechanical Genesis of Tooth Forms," Proc. Phila. Acad., 1878, p. 4.1. 



"[From recent discoveries among the South African Theriodonts it seems more 

 probable that even in the ancestral reptile-mammals the upper jaws bit outside of the 

 lower jaws and teeth, i.e. the jaws were anisognathous. The isognathism of the Dolphins 

 is probably secondary. ED.] 



2 As employed 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 also the parity or disparity in width transversely, from outside to outside," etc. 



It is clear that in the homodont condition, with the teeth simply piercing the food, the 

 greatest comminution (of the food) is effected by isoguathism ; 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. 



