20S BULLETIN 16 9, UNITED STATES NATIONAL MUSEUM 



fissipedes, they are extremely primitive, much more so than any 

 Tertiary dogs or other true fissipedes. The anomaly is thus not so 

 striking as might appear at first sight. Furthermore, within the family 

 the Paleocene forms are distinctly more primitive than Eocene Viver- 

 ravinae and are certainly not ancestral to the Aliacinae, so that com- 

 parison with the latter is misleading. On a small scale, within the 

 family, the Viverravinae are a miniature "archaic" radiation of 

 Miacidae and the Miacinae a later "progressive" radiation, much as, 

 on a far grander scale, the peculiarly specialized periptychids are an 

 archaic radiation and the basically more primitive hyracotheres are 

 a progressive radiation among the ungulate cohort. 



There are two distinctive genera of miacids in this fauna, Didymictis, 

 evidently an abundant form with several species and long known 

 from the distant Torrejon (as well as from numerous later horizons), 

 and Ididopappus, a rarer type known from only two specimens in this 

 fauna and as yet unknown elsewhere. 



Subfamily ViVERRAViNAE Matthew, 1909 (Viverravidae Wortman 



and Matthew, 1899) 



Viverravus, Didymictis, and Ididopappus evidently form a closely 

 related group characterized, among other features, by the prominent 

 anteroexternal cuspule of P*, the elongate oval outline of M^, and the 

 absence of M^3 (Matthev/, 1915). For tliis group the name Viverra- 

 vinae is available, contrasting with the typical miacids, the Miacinae. 

 As mentioned below, Didymictis may be a compound genus, but if so 

 its components are very closely related. Viverravus, also, is a some- 

 what doubtfully bounded genus. Its earlier, lower Eocene species 

 are very close to Didymictis, while some of its later, middle Eocene 

 species, perhaps including the genotype, are so markedly advanced 

 over the early forms that they might not ordinarily be placed in the 

 same genus. Tliis point is not here apropos, but Viverravus is of some 

 present interest because of the possibiUty of special relationship to 

 Ictidopajjpus. 



Viverravus and Didymictis were separated by Matthew (1915) on 

 the basis of the crested heels of Mi_2 in the former, basined in the 

 latter. In fact the early species of Viverravus (e. g., V. acutus, V. 

 politus, and even the slightly later V. dawkinsianus) do have basined 

 talonids, although they are obviously becoming crested by emphasis 

 of the hypoconid and marked reduction of the entoconid. In Icti- 

 dopappus the talonids are quite as basined as in Didymictis, a condition 

 doubtless ancestral for Viverravus also but partly or completely lost 

 in species definitely referable to that genus. Aside from the difference 

 in the heel, relatively slight at the beginning of the Eocene, the early 

 species of Viverravus also differ from Didymictis in the longer, lower 

 trigonid of Mi, which is a strildng resemblance to Ictidopappus. 



