OBSERVATIONS ON THK DENTITION OF CHRYSOCHLORIS. 187 



Leopard we see a triconodoiit tooth doubtless foi-med by 

 degeneration, but apparently as a secondary specialisation we 

 find the posterior molars tending to become triangular and to 

 lock into one another. In Chrysochloris we see the stages 

 by which a simple hoplodont tooth at least can become con- 

 verted into a tooth with a triangular crown ; and we seem 

 justified in concluding from all the available evidence that the 

 mammalian lower molars are all derived from a tooth with a 

 trigon of three cusps, to which is added a posterior talon. 

 But whether the trigon represents the three cusps of the 

 triconodont type, or whether the talon represents the posterior 

 one, does not seem clear. Personally, I incline to the latter 

 alternative. From such a tooth, however, probably all later 

 mammalian lower molars have been derived, even those of 

 the multituberculates. 



In endeavouring to trace the upper molars of higher 

 mammals from a triconodont type, the difficulties are greater 

 than with the lower. We may, however, I think conclude, 

 with Gidley, that the evidence is very strongly against the 

 antero-internal cusp being the homologue of the reptilian 

 cone, and in favour of the antero-external cone being the 

 homologue. Embryology is said to support this view, but I 

 am by no means satisfied that the evidence is valid, and 

 therefore give no weight to it. But if we accept the pi'e- 

 molar-analogy theory, we must conclude that the primitive 

 cusp is the antero-external. The evolution of the other 

 cusps is, however, a matter of considerable uncertainty. 



If the upper molars and premolars are derived from a tri- 

 conodont type, it is almost necessary to assume that the 

 anterior cusp became reduced or disappeared. The upper 

 molars of Dicrocynodon, when viewed from the outside, 

 show only the main cusp and the small posterior one, and 

 only two cusps are generally found on the outer side of the 

 Cretaceous upper molars. These two cusps become in later 

 molar types the paracone and metacone. The antero-internal 

 cusp has developed as an internal heel to meet the posterior 

 cusp of the lower molar. Fig. 9 represents the cusps and 



VOL. 2, PART 1. 10 



