rORT UNION OF CRAZY MOUNTAIN FIELD, MONT. 207 



7MIMOTRICENTES species 



U.S.N.M. no. 6178 is a left Mj from Loc. 9, at a low level in the 

 formation. It apparently belongs in this or a closely allied genus. 

 Its dimensions, 6.2 by 5.2 mm, are perhaps not significantly different 

 from those of M. latidens, although it is relatively somewhat narrower. 

 The trigonid is distinctly longer than in the type of the genus and 

 about equal to the talonid in size, not distinctly smaller as in the type. 

 Although inadequate for identification, the occurrence merits mention 

 in view of the horizon represented. 



Family MIACIDAE Cope, 1880 



The known history of the Miacidae is anomalous and emphasizes 

 the inadequacy of some of our knowledge of details in this early epoch 

 and the probably erroneous character of some negative conclusions 

 regarding it. The miacids (so carefully and fully defined by Matthew 

 in many works that diagnosis here is unnecessary) are a specialized 

 group, for in them the carnassial shearing teeth are very well devel- 

 oped, despite their absence in all other known Lower and Middle 

 Paleocene mammals. Furthermore, they are an adaptive and potent 

 group, for their carnassials are Mi and ?•*, as in the Carnivora (vera) 

 or Fissipedia and there is every reason to believe that they are, in a 

 broad sense, ancestral to all the latter. The appearance of this ap- 

 parently modernized group in a fauna otherwise almost wholly archaic 

 is extraordinary. 



The known distribution within the famil7 is also noteworthy. The 

 first genera to appear, and the only ones known before the true Eocene, 

 are not the most primitive and generalized and are not ancestral, 

 even structurally, to the majority of later types. All have lost M^a, 

 unquestionably present in the ancestry. Even aside from the fact 

 that these teeth are present in most later miacids, they are almost 

 universall}?" present in Middle Paleocene mammals of other famihes. 



This anomalous history must involve, first, rapid progressive evolu- 

 tion of the group generally, the Miacidae, which is not surprising in 

 view of later history, wliich shows this general type to be probably 

 the most plastic and adaptive of all mammals. Second, it must involve 

 the early, minor differentiation of a special fine, the Viverravinae, 

 which entered the regions known to us paleontologically at about the 

 beginning of the Middle Paleocene, while the more varied adaptive 

 Miacinae were confined, until the great Eocene invasion, to some 

 facies or region still unknown to us. 



In speaking of the Miacidae as specialized, it is important to em- 

 phasize the relative value of the words. They are specialized in com- 

 parison with the extraordinarily archaic contemporaneous Arctocyo- 

 nidae, which are not far from being generalized primitive placentals, 

 but in comparison with the other carnivores, specifically with the 



