332 O. C. Marsh—American Jurassic Mammals. 
CTENACODON. 
The genus Ctenacodon was based upon lower jaws, one of 
which is represented in Plate VIII, figure 1, and others, in 
figures 4, 7, and 8. The single, long, pointed incisor, the four, 
compressed, cutting premolars, and the two, minute, tubercular 
molars, form together a peculiar dentition. The long, sharp in- 
cisor shows no signs of wear whatever, and hence could not be 
opposed to the large upper incisor of AJdlodon. Its position 
was close to its fellow, and the two evidently acted together, as 
indicated in figure 9, Plate VIII. The four premolars form a 
close-set series, with their upper margins on a curve, all more or 
less notched. Some specimens, at least, show distinet marks of 
wear on the outer sides of the crowns, which are sometimes 
worn to a uniform surface. The two small tuberculate molars 
are of the Wicrolestes type, with a deep longitudinal groove on 
the upper surface of the crowns. 
The entire upper dentition of Ctenacodon is not known with 
certainty, but it probably corresponded in its main features to 
that of Allodon. A portion of the upper jaw, with typical pre- 
molars, is shown in Plate VIII, figures 2 and 3. The posterior . 
premolars, especially the last two, show strong marks of attri- 
tion on the inner sides of the crowns, and these were opposed 
to the compressed premolars below, forming together a most 
effective apparatus for cutting. 
Some of the lower jaws at present referred to Ctenacodon 
apparently show no signs of wear on the premolars, and as the 
large incisor is not preserved, it is impossible to say definitely 
that they may not pertain to Adlodon. It is likewise quite 
probable that some of the lower jaws considered as Plagiaulax 
may belong with some of the specimens now known as Golodon. 
The exact correlation of the two forms cannot be determined 
with certainty until the upper and lower jaws are found together 
in position. 
Ctenacodon may be distinguished from the type of Plagiau- 
lax (P. Becklesiz) in having four premolars instead of three. 
The summits of these teeth alone are notched, and the sides 
smooth, not obliquely grooved as in Plagiaulax. The condyle, 
moreover, is separated from the angle of the jaw, not confluent 
_ with it. Ctenacodon, also, has the angle of the jaw not only 
strongly inflected, but its outer margin efflected into a wide 
horizontal shelf, making this one of the most peculiar features 
of the genus. 
The vertical posterior condyle in Ctenacodon implies a strong 
post-glenoid process, that would confine the jaw to a vertical 
motion. 
