rORT UNION OF CRAZY MOUNTAIN FIELD, MONT. 



209 



Were it not for the unusual proportions and simple structure of P4, 

 Ididopappus would make an ideal ancestor for Viverravus. Some 

 species of the latter, e. g.,V. acutus, have P4 small and simple, but much 

 more trenchant than in Ididopappus, stronglj^ compressed laterally, 

 the posterior cusps well developed, and all cusps in a straight line, not 

 subtriangular. The Ididopappus P4 might be ancestral to this, but 

 there is no good evidence that it was, and Viverravus may after all be 

 an offshoot of Didymidis in which the trenchant P4 was already highly 

 developed in the Middle Paleocene. 



Genus DIDYMICTIS Cope, 1875 



This exceptionally long-lived genus is recorded from all levels from 

 the Torrejon to the end of the lower Eocene. During this period 

 there is not only definite evolutionary advance but also evidence of 

 the presence of several difi'erent phyla. It is quite possible that one 

 or more of these can be and should be separated generically, but the 

 criteria for doing so are poor at present. A useful step was Matthew's 

 (especially 1915) revalidation of Viverravus, which was long confused 

 with Didymidis or considered a synonym. Now Matthew (Pale. 

 Mem.) has further separated the sole Torrejon species, D. hay- 

 denianus, and placed it in a new subgenus, Didymidis (Protidis), with 

 the suggestion that this may prove to be of generic rank. 



The most abundant species in the present collection, D. microlestes, 

 introduces some dijEculty in this arrangement. In its more important 

 morphological characters it compares with the types of Didymidis 

 {Protidis) and of Didymidis (Didymidis) as shown in table 46. 



