330 Bulletin American Museum of Natural History. — [Vol. 92. % A 
hypoconulid, and usually with small accessory cusps on both the atiterior 
and posterior slopes of the metaconid and on the anterior face of the ento- 
conid; hypoconid and metaconid separated by a valley; last lower molar 
similar to anterior ones but with the hypoconulid enlarged and produced 
into a short heel, the tooth being usually the longest and always the narrow- 
est of the lower molars; posterior upper and lower premolars with a strong 
tendency to become molariform; first lower premolar single-rooted; first 
EctTocion 
Lower EKocene 
_PHENACODUS 
Lower Eocene 
TETRACLANODON 
Basal Eocene 
, 
Fig. 1. Outlines of upper cheek-teeth of the three genera of Phenacodontide. 
upper premolar two-rooted in some forms; canine of moderate size; incis- 
ors broad, chisel-edged. | 
Occurrence. Tetraclenodon is confined to the, Torrejon ! where it is the 
most abundant form, specimens of the genus constituting about one-fifth 
of the entire collection from this horizon. Phenacodus and Ectocion range 
together throughout the Lower Eocene, from the Clark Fork to the Lost 
1A closely related, possibly identical, genus is found in the Fort Union beds in 
Montana. 
. 
