rro 



don, in wbicli the form of the mandible also forbids a reference to the 

 Marsupialia, as Gervais has remarked. Both genera are doubtless mem- 

 bers of the sub-order of Creodonta. The genus Eymiodon, on the other 

 liand, is not referable to the same group, for I find in a specimen of the 

 H. requieni from Desbruges, preserved in the Museum of the Jardin des 

 Plantes, that the scaphoid and lunar bones are coossiiied. Moreover the 

 figure given by Professor Gervais* representing the brain of the originally- 

 described t^pe, H. leptorliynclius of the Miocene period, displays characters 

 of the true Garnivora. The anterior part of the cranial cavity of the 

 specimen molded is broken away. 



It is possible that the genus Diacodon Cope belongs here also ; its spe- 

 cies resemble slightly the Marsupialia in the inferior dentition, and are of 

 small size. 



The genus Mesonyx,\ which I discovered in the Bridger beds of Wyo- 

 ming, has the trochlear face of its astragalus completely grooved above as 

 in the true Ga,rnivora, and its distal end presents two distinct facets, one 

 for the cuboid, and the other for the navicular bones It represents on 

 this account a peculiar family, the MesonychidcB. 



There are various degrees of development of the sectorial structure of 

 the molars in this sub-order. In some of them, as Bidymictis, only one of 

 the inferior molars presents this structure ; in others two, and in others 

 three. In one type, the last superior molar is longitudinal ; in others, it 

 is transverse. In Arctocyon the superior true molars are tubercular. 



I have heretofore:}: defined three families, the Amblyctonidce, the Oxyos- 

 nidm, and the ArctocyonidcE. I now add the Miacidm and Mesonychidce. 

 The definitions are as follows : 



I. Ankle joint plane transversely. 



True molars above and below tubercular; last superior not 



transverse Arctocyonidm. 



Superior true molars tubercular; first inferior "tubercular- 



sectorial " MiacidcB. 



Last superior molar trenchant, transverse ; inferior true 



molars tubercular-sectorial Oxymnidm. 



Last superior molar longitudinal ; inferior true molars 



without developed sectorial blade Amhlyctonidm. 



II. Ankle joint tongued and grooved, or trochlear. 



Inferior molars each with one conic tubercle and a heel ; no 

 developed sectorial blade MesonycMdm. 



I now give the characters of the genera. All these are derived from ex- 

 amination of typical specimens. The opportunity of doing this I owe to 

 the kindness of Messrs. Leidy, Gervais, Gaudry, Filhol and Lemoine. 



* Loc. cit., pi. vi, flg. 5. 



t Ann. Rept. U. S. Geol. Surv. Terrs., 1872, p. 550. 



I Report Capt. G. M. Wheeler's Expl. Surv. W. 100 Mer , 1877, iv, pi. 11, p. 89. 



