46 Wortman — Studies of Eocene Mammalia, etc. 



speculation. If we now turn our attention to the case before 

 us, we observe in Dissacus the structure of the molars to be 

 so astonishingly like that of the fourth premolar, that to my 

 mind there is not the faintest shadow of a doubt that the 

 postero-external and internal cusps have been added to the 

 primitive cone in exactly the same mariner, and precisely the 

 same order, as they have been in the premolars. The signifi- 

 cance of the disparity in size between the external cusps, just 

 as in the fourth premolar, is explained by the fact that the 

 postero-external cusp is in an incomplete or transitional stage 

 of development. It does not seem reasonable to me that any 

 other interpretation can be placed upon it. The truth of this 

 assertion and the correctness of this interpretation become 

 apparent at a glauce, when we compare them with the molars 

 of Mesonyx, for in the latter, as we have already seen, the two 

 external cusps are equal in size. That they became so in the 

 course of, and as a result of, a long series of modifications 

 reaching over nearly the whole of Eocene time, establishes 

 beyond any possibility of doubt the further fact that in this 

 phylum we are dealing, not with a degenerating molar crown, 

 but with one which was progressively increasing in complexity. 



It will thus be seen that in this case, the evidence is over- 

 whelmingly in favor of the view that the addition of the two 

 cusps in question has taken place in a manner very similar to, if 

 not absolutely identical with, that of the premolars, and equally 

 opposed to the view embodied in Osborn's theory of rotation 

 or migration. If the disparity in size between the external 

 cusps of the molars of Dissacus thus finds a true and satisfac- 

 tory explanation, the similar condition of these cusps of the 

 first and second molars of Viverravus, Oodectes, Vulpavus, 

 JVeovulpavus, Uintacyon, Prodaphwnus, and many others, 

 must be interpreted in the same way. It therefore follows by 

 inference that the tri tubercular molar in the entire order 

 Carnivora has been formed in this manner, and not by any 

 supposed rotation or shifting of the cusps, as assumed by 

 Osborn. If in the Carnivora the tri tubercular crown has been 

 formed in the same way that it has in the premolars, what 

 shall we say of the other orders in which we do not have the 

 first vestige of evidence beyond that afforded by the premolars ? 



I have deemed this matter of sufficient importance to go 

 into it thus fully for the reason that a very elaborate and com- 

 plicated system of nomenclature has been built up by Osborn 

 upon what I fully believe to be an erroneous foundation. The 

 manner of origin of these cusps having been incorrectly deter- 

 mined, it follows that the homologies are wrong, and the 

 names applied inappropriate and misleading. They should 

 therefore be abandoned, since they can be productive only of 

 confusion and error in any attempt at further progress in the 

 subject. 



[To be continued.] 



