CHAP. Vl] THE DENTAL SYSTEM OF THE PRIMATES 



285 



(iv) Assuming that lateral denticles had been added to a 

 primitive cone, the next search was directed towards identifying 

 the original cusp in the upper and lower teeth respectively. Cope 

 (followed by Osborn in this particular) suggested that the "lateral 

 denticles " were " rotated " from the sides of the original cone, and 

 passed into new positions (Fig. 205). 



An important point is that the rotation was supposed to have 

 occurred in precisely opposite directions in the upper and lower 

 teeth respectively. In the upper and lower teeth alike the cusps 

 came to rest at the angles of a triangle (cf. Fig. 205 D), but the 

 arrangement differed slightly in the upper and lower series inas- 



Md 



<E^X> 



Fig. 205. A and B, diagrams of lower molar teeth to illustrate the process of cusp 

 rotation. C and D, diagrammatic representations of the lower molar teeth of 

 ancient fossil mammals bearing a similar relation to each other. C, tooth of 

 Dromatherium, D, tooth of Peralestes. (After Osborn.) 



much as in an upper tooth two cusps were external (buccal) and 

 one cusp internal (or lingual) in position, while the cusps of the 

 lower teeth were placed so that one was external (buccal) and two 

 internal in position. This part of the theory is very important, 

 for the identification of the original cone (in a given tooth) 

 depends directly upon it. And consequently, identification is still 

 possible even when the original cone has lost its superiority in 

 point of size. The final relations of an upper and a lower molar 

 tooth are shewn in the following figure (Fig. 206). 



(v) The same illustration (Fig. 206) reveals another point of 

 importance, viz. that the teeth alternate in the upper and lower 



