BUNOTHERIA. 
75 
extent. The heel is large and without prominent tubercles. In the 
Miistelidce, the inner of the two median cusps is often reduced to a rudi¬ 
ment, or is entirely wanting, and the heel is large. The lower sectorial of 
the Hycmidoi has no" inner tubercle, and the heel is mucli reduced. ■»In 
some of the saber-toothed Tigers, the heel remains as a mere rudiment, while 
in the true Cats it has entirely disappeared, and the carnassial tooth remains 
perfected by subtraction of parts, as a blade connecting tw'O subequal cusps. 
The UycEYiodontidm^ as is known, possess three carnassial teeth without inner 
tubercle. The history of this form is as yet uncertain, as it was evidently 
not derived from cotemporary forms of the Eocene with tubercular secto- 
rials. 
The development of the carnassial dentition has thus been accom¬ 
plished first by an addition of an anterior cusp, and subsequently by the 
subtraction of the inner and posterior cusp, so that of the original four of 
the quadrituberculate molar, but a single one^ i. e., the anterior external, 
remains. 
In addition, these animals exhibit the following characters of the 
skeleton: 
The glenoid cavity of the squamosal bone is transverse, and well defined 
anteriorly and posteriori}^, as in some Carnivora. Of the first series of carpal 
bones of the four genera named, I have been able to learn nothing; but in 
the genus Synoiolotlierinm^ from the Bridger Eocene of Wyoming, which 
probably belongs to this group, the scaphoid and lunar bones are separate, 
and not united as in the Carnivora. In all the genera, the ilium has a well- 
marked external anterior ridge, which continues from the acetabulum to the 
crest, distinct from the internal anterior ridge. The ilium has therefore 
an angulate or convex external face, as in Insectivora and Marsnjgialia, and 
does not display the usual expansion in a single plane of most of the pla- 
centals. In all the genera, there is a strong tuberosity in the position of the 
anterior inferior spine, which is wanting in the Mammalia, excepting certain 
Insectivora undi Frosimia, although it marks the position of the origin of the 
rectus femoris muscle in all types. 
In Ambloctonus, Didymictis, and three undetermined forms, the femur 
supports a third trochanter. 
