216 BULLETIN 169, UNITED STATES NATIONAL MUSEUM 



cusped (worn) and projects almost directly internally, not at all 

 forward, in marked distinction from Didymictis. 



M^~^ have the same cusp structure as in all miacids, but are highly 

 peculiar in proportions, being very short and wide, markedly tri- 

 angular, not at all rounded, with sharply emarginate external borders. 

 These characters are more nearly approached by Viverravus than by 

 Didymictis but sharply distinguish Ictidopappus from any other 

 miacid. 



Measurements of this specimen are as follows: Length P^, 2.6; 

 length P^ 4.8; A\ddth P*, 3.7; length M^ 3.2; width M^, 5.2; length 

 M2, 2.0; width M^, 3.7. 



Family MESONYCHIDAE Cope, 1875 



This family, so widespread elsewhere in the Middle Paleocene and 

 on into the Eocene, is represented in the present fauna only by two 

 broken teeth. 



Genus DISSACUS Cope, 1881 



DISSACUS, species undetermined 



U.S.N.M. no. 9692 from the Gidley Quarry is the base of a tooth 

 with the heel preserved. It agrees in size with Mg of Dissacus nava- 

 jovius, of the Torrejon, but differs in having a very vestigial basin 

 internal to the heel crest. Another specimen, from the Gidley Quarry, 

 is perhaps Mi, lacking the anterior portion. The metaconid is very 

 small and is relatively anterior, as in some species of Dissacus, but the 

 heel is relatively shorter than in other specimens of that genus known 

 to me. It is clear that these fragments are mesonychid, and there is 

 nothing to distinguish them certainly from Dissacus, but they are not 

 really identifiable. 



Order CONDYLARTHRA Cope, 1881 



As with most of the major groups here considered, Matthew has 

 thoroughly reviewed the taxonomic history of the order Condylarthra. 

 Since, however, I propose a marked change in the current arrange- 

 ment of the order, an outline of tliis liistory must now be given. Cope 

 proposed the name Condylarthra (then supposed to be a suborder of 

 Perissodactyla) in 1881, basing it on the Phenacodontidae and princi- 

 pally on Phenacodus but with Peripiychus ("Catathlaeus"), Mioclaenus, 

 and Tetraclaenodon ("Protogonia") probably and Anisonchus possibly 

 included. Subsequently (1882-1884) the family Periptychidae was 

 distinguished, but retained in the Condylarthra, the family Menisco- 

 theriidae was added, and Mioclaenus was excluded, being considered 

 first an artiodactyl, then a creodont. On the latter point, Scott 

 (1892) showed that many species placed in Mioclaenus by Cope were 



