EVIDENCE BEARING ON TOOTH-CUSP DEVELOPMENT IO5 

 SUMMARY AND CONCLUSIONS. 



Summing up the evidence derived from this preliminary 

 study, the following conclusions are suggested : 



1. That the evidence obtained from the Mesozoic mammal 

 teeth furnishes no support to the tritubercular theory in so far 

 as it involves the position of the protocone and the derivation of 

 the trigonodont tooth from the triconodont stage through the 

 shifting of the lateral cones outward in the upper molars and 

 inward in the lower molars. 



2. That it supports entirely the embryological evidence that 

 the primary cone is the main antero-external cusp, or paracolic, 

 having retained its position on the outside in most upper molars 

 (see exceptions above, p. 95). 



3. That it agrees in the main with Huxley's " premolar- 

 analogy " theory, as supported by Scott. 



4. That the molars of the Multituberculates, Triconodoti, 

 Dryolestes and Dicrocynodon, were apparently derived inde- 

 pendently from the simple reptilian cone ; hence the supposi- 

 tion follows that the trituberculate type represents but one of 

 several ways in which the complex molars of different groups 

 may have been derived. 1 



5. That in the forms derived from the trituberculate type of 

 molar the order of succession of the cusps is not the same in all 

 groups, and apparently homologous elements are sometimes de- 

 veloped from different sources. Hence it follows that no theory 

 involving an absolute uniformity of succession in the development 

 of complex molars zu ill hold true for all groups of mammals. 



In the foregoing pages I have restricted the use of Osborn's 

 tooth-cusp nomenclature for the reason that, in this particular 

 discussion, there are some cases in which it is not strictly appli- 

 cable and might lead to confusion. 



On similar grounds Dr. Wortman 2 has expressed the opinion 

 that all attempts to establish a tooth-cusp nomenclature founded 

 on supposed homologies are "foredoomed to failure" and 

 should be entirely abandoned as " useless and confusing." I 



1 Somewhat similar conclusions have been reached from different reasoning 

 by E. S. Goodrich, M. Tims and others. 



2 Amer. Journ. Science (4), Vol. 16, 1903, 265-368. 



Proc. Wash. Acad. Sci., August, 1906. 



