li)l().] Evolulion of the Caniassial Irct/i; the Miacidce. 309 



derivation of the Caniivora Fissipedia largely depends upon one's eonce])- 

 tion of the nature and limits of that group, whieh, as shown by the labors 

 of Cope, Marsh, Scott, Osborn, Wortman, Matthew, and several European 

 paltieontologists, is certainly an offshoot of the stock that gave rise to the 

 higher Creodonts. 



The dentition of the typical Fissipeds such as the civet, the dog, the cat, 

 and the marten, represents a high stage of evolution of the tritubercular 

 tyjje. In the Jurassic 'Frituberculates the small antero-posteriorly narrow 

 upper molars were as many as six or eight in numl)er on each side, and the 

 premolars were compressed and conical. In the most primitive carnivorous 

 Marsupials, \\hich j)robably represent the Cretaceous stage, the molars are 

 reduced to four but the premolars are still simple; in the Basal Eocene 

 Oxyclsenidc-e, which are very primitive Placentals, perhaps Insectivores or 

 ancestral Creodonts, the tritubercular molars are reduced to three in num- 

 ber and the fourth premolar has become bicuspid. In the Eocene Creodont 

 families Arctocyonirla? and Mesonychidiie, the center of gravity, so to speak, 

 of the dentition still lies in the large crushing upper molars; in the Hya^no- 

 dontidse the molars become compressed and shear-like. But in the Oxysen- 

 idpe the center of the dentition is shifted forward by the loss of the posterior 

 upper molar, and the shearing function in the upper jaw is predominant on 

 the postero-internal extension of the first molar. 



In the Basal Eocene Didi/mictis (Viverravus) haydenianus Cope (1884, 

 pi. xxiiie, figs. 12, 13) a specialized forerunner of the Miacidse (Viverravidse), 

 the center of the dentition is shifted still further forward, since the shearina: 

 function is concentrated on the postero-internal blade of p^ and on the antero- 

 internal blade of n\, while the post-carnassial teeth are much reduced, only 

 the small m' remaining. Finally, in the Felidne, the center of dentition 

 reaches its anterior limit, since p^ alone is carnassial and of large size, m^ 

 and m^ are absent and m^ is reduced to a vestige. 



In Cope's original definition (Palaeontological Bulletin, No. 20, 1876, pp. 

 1-4; Tertiary Vertebrata, p. 251) Didymicii.s, and consequently by implica- 

 tion the family it represents, were included in the Creodonta. The char- 

 acters used to define the group also clearly include Didymictis. 



However, the character of the carnassials in Didymiriis antl the Miaciche 

 so sharply separates them from the Arctocyonida^, Mesonychidjie, Oxytenidjie 

 and Hyfenodontida' and allies them with the Fissipedia that Wortman 

 (1901, pp. 337-338) has grouped the MiacidiB ("Viverravidi^e") with the 

 Fissipede Carnivora under the subordinal term "Carnassidentia." It is 

 also shown by Matthew in his recent memoir on the Bridger Carnivora 

 (1909, p. 353) that the Miacidse include several divergent lines of specialization 

 some of which seem to be directly ancestral to the modern Carnivore families. 



