478 DE. H. W. MAEETT TIMS OK THE 



not always participate to the same extent. If, for instance, tlie 

 carnassial teeth of the Tiger, Dog, and Bear be compared, this 

 fact at once becomes apparent. And since I regard the Posterior 

 Cingulum-cusp as a structure of greater antiquity than the 

 Secondary cone, I conclude that the greater the share which the 

 Posterior Cingulum-cusp takes in the formation of the Metacone, 

 the earlier will that Metacone be developed, and viee versa. 



Such is, I believe, the explanation to be given of the somewhat 

 varying results obtained by the aforementioned observers. 



I have, I hope, said sufficient to indicate the main points of my 

 Theory of Cingulum-cusp development. I do not say that there 

 may not be many objections to it, but I think that it is, at any 

 rate, free from serious ones. 



The points in its favour may be thus summarized : — 



(i.) It harmonizes more fully with what is known of the 

 development of the teeth than either the Tritubercular 

 or Multitubercular theory, the Primary cone repre- 

 senting the Reptilian cone and being always present, 

 (ii.) It is quite possible and easy thus to homologize the 

 cusps of all teeth, except perhaps those derivative of 

 the Multituberculate type, 

 (iii.) It is in accordance with Palseontological history. 

 (iv.) No supposed rotation of cusps is required to have taken 

 place. 

 It is probable that in time, with greater knowledge and ex- 

 perience, many of the points of detail will have to be modified ; 

 indeed, I wish now only to give an outline of this hypothesis in 

 order that it may be more generally tested. Great difficulty has 

 been found in endeavouring to write a lucid explanation of this 

 view, owing to the impossibility of homologizing the cusps under 

 the old terminolegy. 



In conclusion, I most gratefully express my thanks to Prof. Gr. 

 B. Howes, not only for having suggested this subject for investi- 

 gation, but also for having enabled me to carry it out in the 

 Laboratory under his charge and for much kind advice and 

 criticism. I would also express my thanks to Mr. Gr. L. Parsons, 

 of the "Westminster Medical School, for having made the 

 drawings necessary for illustration of this paper. 



