EVIDENCE BEARING ON TOOTH-CUSP DEVELOPMENT 95 



metastyle have drawn closer together and compose the entire 

 outer portion of the crown, while the paracone and metacone 

 are closely approximated, forming the greater part of the inner 

 portion of the crown, the protocone being very much reduced. 

 Thus in Harfioccphalus a stage is reached nearly analogous 

 to that of Potamogale^ the principal difference being that the 

 metacone is the dominant cusp instead of the paracone, as in 

 the latter genus. 1 



From these comparisons it seems reasonably clear that such 

 forms as Centctes, Ericulus and Chrysochloris have attained a 

 secondary or pseudo-tritubercular form by passing through some 

 such stages of evolution as are suggested by the two series here 

 selected. Other examples of a fusing paracone and metacone 

 and reducing protocone may be found in the molars of some of 

 the creodonts and carnivorous marsupials and in the sectorials 

 of many of the carnivores. 



From the foregoing it now seems to be demonstrated beyond 

 question that the main inner cone of Centetes and Ericulus is 

 not the protocone as observed in normal groups, but, if not 

 entirely made up of the primary cusp (paracone), it at least in- 

 volves that element and Woodward's contention that the evi- 

 dence of embryology is in entire harmony for the molars and 

 premolars is not controverted by these seeming exceptions as 

 supposed by Osborn. 



Wortman of late has strongly opposed what he terms the 

 "cusp migration theory," and has brought considerable evi- 

 dence to show r that, in the creodonts and carnivores, at least, 

 the cusps of the upper molars in general are homologous to 

 those of the molariform premolars and have had substantially 

 the same history in their development. 



Against this combined evidence Osborn 2 has recently re- 

 affirmed the tritubercular theory, " as originally proposed," 

 resting the whole question on the point of evidence as to 

 " whether the main reptilian cone, or protocone, of the ances- 



1 In the Laramie mammals I find that the metacone equals or is larger than 

 the paracone in those forms in which the postero-external heel is well developed 

 in the upper molars. 



2 Amer. Journ. Science (4), Vol. 17, 1904, 321-323. 



