318 PROCEEDINGS OF THE ACADEMY OF [1892. 



the junction of the pterygoid plates of the alisphenoids. The cra- 

 nial foramina are very much as in the dogs, but there appears to be 

 one important and constant difference between the American and 

 European species of the genus, in that the latter possess an alisphe- 

 noid canal, which the former have not. In the American forms the . 

 horizontal ramus of the mandible is long and slender, gradually 

 deepening posteriorly, and forming a very long symphysis with its 

 fellow ; the coronoid is more or less triangular in shape and the con- 

 dyle is placed very low ; the angle terminates in a short hook. 



The skeleton, which is very con)pletely known, differs from that 

 of Oxycena only in details and need not be described here, far- 

 ther than to mention the large articulation between the fibula and 

 the calcaneum. The American hya?nodonts have the scaphoid, 

 lunar and central bones all separate, while in the European species 

 they are coalesced. 



Four American species of Hycenodon have been described : H. 

 horridas Leidy, H. crucians Leidy. H. cruevtns Leidy, H. lepto- 

 cephalus Scott, all from the White River. 



MIACIDJE Cope. 



Fourth upper premolar and first lower molar forming the single 

 pair of sectorials ; superior molars tritubercular, inferior molars, 

 except m^, also tubercular. 



The members of this family which form the connecting link 

 between the Creodonta and the Carnivora, so far at least as their 

 dentition is concerned, are, unfortunately, as yet very imperfectly 

 known, no complete skull or well })reserved feet having been 

 discovered. 



DIDYMICTISi Cope. 



I| Ct pi Ml. The anterior upper premolars are simple, com- 

 pressed and trenchant cones, without accessory cusps, but P4 is a 

 fully developed sectorial, the proto- and tritocones forming the 

 blade ; the protostyle, which occurs in the cats and viverrines, is also 

 indicated. M^ is quite large, with the three primary cusps present, 

 though the raetacone is much reduced and the metaconule has dis- 

 appeared, and there is no hypocone ; the antero-external angle oi 

 the crown is greatly extended and there is a very broad cingulum. 



1 Syst. Cat. Vert. Eocene N. M., U. S. Geogr. Surv. W. IdOth Mar., 1875, 

 p. 11. 



