Ti:rrrr,KKrriA i.\ PRIMATKS 63 



exceptionally ////////, rr//A//'. This fact was pointed out l>y Professor 

 Cope in his article entitled. " Leniurine Reversion in Human Dentition." 



Up to a certain point in their evolution the molar teeth of all 

 mammals followed exactly the same route.* It follows that if we once 

 grasp the principles of cusp addition upon this triangular ground 

 plan we can compare the cusps of the molars of man with those of 

 any other mammal. In the teeth of the bear, for example, the 

 homology is very obvious indeed. But in the teeth of the cat the 

 homologies can only be determined when we procure the ancestral 

 forms of cats, for in the evolution of the large sectorials many 

 cusps have degenerated. Some years ago, when I had fully demon- 

 strated the truth of Cope's theory by my own studies, I saw the 

 importance of using a set of standard terms for the cusps. These 

 have since been almost universally adopted by comparative anato- 

 mists, but have not, as yet, I believe, made much headway among 

 human odontologists. They are, as follows, as applied to the human 



teeth : 



UPPER MOLARS. 



Anterior palatal . . . Protocone ) 



Anterior buccal . . . Paracone , Primitive triangle, or " trigon." 



Posterior buccal .... Metacone ^ 



Posterior palatal .... Hypocone Primitive heel, or "talon." 



LOWER MOLARS. 



Anterior buccal Protoconid In- ,. 



. , , . }- Primitive triangle, or " trigonid. 



Anterior lingual . . . Metaconid J 



Posterior buccal . . . Hypoconid ) 



Posterior lingual .... Entoconid > Primitive heel, or " talonid." 



Posterior mesial .... Hypoconulid j 



When we understand that all the teeth of all mammals have this 

 key, this tritubercular key, we c'an unlock the comparisons through 

 the series and point out the homologies. 



There is further evidence in support of the theory of cusp addition 

 which I will now briefly mention. It is that brought forth by tin- 

 very investigations of Dr. Carl Rose, which he has used to support 

 the concrescence theory. We should expect, in the embryonic jaw 

 that the calcification of the tooth-germ would be very significant, 

 because we know that the embryonic structures in their development 

 follow the order of addition or evolution. The order of evolution is, 

 to a certain extent, repeated in embryonic development. How is it 

 with the teeth ? Dr. Rose has given a most exact account of the 

 mode of calcification of the tooth-germ within the jaw ; this is also 

 now to be had in the form of wax models, prepared by Professor 

 Zeigler, of Freiburg. 



1 Journal of Morphology, Vol. II., 1888, pp. 1-24. 



* [At the present time this statement seems very doubtful. -ED.] 



