1S81.J 1'" [Cope. 



I. Last inferior molar with oval outline; heel small; anterior inner 

 cusp simple. 



Size medium ; length of true molars .025 ; depth of ramus at M. II, .018. 



P. apternus. 



II. Last inferior molar wedge-shaped, with heel prominent ; anterior 

 inner cusp simple. 



Large; true molars .041 ; P-m. IV .014 ; depth of ramus at M. II, .037. 



P. primcBvus. 



Medium ; true molars .027 ; depth at M. II .017 ; last molar smaller 



P. vortmani. 

 Smaller ; true molars .022 ; depth at M. II .013 ; last molar elongate ;. . 



P. macroptermos. 

 Smaller ; last four molars .027 ; P-m. IV .007 ; depth at M. II .013 ; last 



molar with short heel P. bracliypternus. 



Smallest ; true molars .017 ; depth at M. II .012 ; heel long ; cusps ele- 

 vated • .■ P. zunicnsis. 



III. Last inferior molar wedge-shaped, with prominent heel ; anterior 

 inner cusp double ; 



Least ; last inferior molar .006 ; heel narrow ; true molars (superior) .016. 



P. laticuneus. 

 Two other species have been described, the P. sulcatus, and P. omnivorus 

 Cope. The former I suspect belongs to another genus. I am not now 

 sure of the distinctness of the latter from P. primcBmis. 



Phenacodus hemiconus, sp. nov. 



Represented by the posterior two superior molars of an individual in- 

 termediate in size between the P. primcEvus and P. puercensis. The pos- 

 terior molar is peculiar in the very rudimental character of the posterior 

 internal lobe, which is reduced to a mere wart on the cingulum. The 

 posterior external tubercle is also rudimental, not exceeding the posterior 

 inner in dimensions. The anterior tubercles, including the intermedftite, 

 are well developed, the internal exceeding the external. The cingulum 

 is wide and crenate, and is only wanting on the external base of the crown. 

 The penultimate molar does not differ so much from that of P. primmmis, 

 but the two internal cones are not so deeply separated at their base. The 

 tubercles are all but little worn, and are conical in form, the external 

 flattened on the external faces. Enamel wrinkled. 



Measurements. M. 



Diameters of M. II \ ^^^teroposterior 009 



( transverse 012 



Diameters of M. Ill \ anteroposterior 010 



( transverse 013 



The size of this species precludes the possibility of its identity with any 

 of the other species described here. 



Phenacodus wortmani Cope. Bulletin IT. S. Geol. Surv. Terrs, vi, 

 1881, p. 199. Hyracotherium vortmani, American Naturalist, 1880, p. 747. 



PliOC. AMEK. PHILOS. SOC. XX. 111. W. PRINTED MARCH 16, 1882. 



