eou Bulletin American Museum of Natural History. [Vol. XXXIV, 
of the tooth by a ridge descending from the metaconid, thus forming a loop 
in front of the anterior pair of cusps. 
Previous to the beginning of the Lower Eocene exploration in 1910 the 
collection contained but three specimens of the genus and one 
other fragment from the Gray Bull beds, and a lower Jaw from the Lost 
Cabin. The collection now numbers over 150 specimens, chiefly jaw frag- 
ments and isolated teeth. The genus is confined to the Wyoming hori- 
zons. It is most abundant in the Clark Fork, very common in the Sand 
Coulee and the Gray Bull, absent from the Lysite and represented in the 
Lost Cabin by a single specimen. In its geological distribution it nearly 
parallels that of the large Phenacodus, P. primevus. 
E. superstes 
(Type, Lost Sabin beds, Wind 
River basin, Wyo.) 
EH. osbornianus 
(Topotype, Gray Bull beds, Big- 
horn basin, Wyo.) 
E. ralstonensis 
Ces (Type, Clark Fork beds, Clark 
a Alo: /6050, A.M, Fork basin, Wyo.) 
; E. parvus” 
es eyes a (Type, Clark Fork beds, Clark 
Pye. /6080, A.M. Fork basin, Wyo.) 
Fig. 13. Lower teeth, external views, of the species of Ectocion. 
‘ A single specimen from the Sand Coulee beds has a few skeleton frag- 
ments in association, of these an astragalus is the most important. In 
general it is intermediate between Tctraclenodon and Phenacodus. It 
resembles Tetraclenodon in thé long neck, in the narrow ectal facet and in. 
having the sustentacular facet broad in the middle and tapering rapidly 
above and below, the outer line being angular instead of curved as in 
Phenacodus. The astragalus resembles Phenacodus in not being flattened 
and in having no astragalar foramen.! The lower end of a radius shows very 
small facets as in Phenacodus. In Tetraclenodon these are relatively larger. 
The species of Ketocion present as difficult a problem as those of Phena- 
1 This foramen is usually absent in Phenacodus but not always, 
