336 O. C. Marsh—American Jurassic Mammals. 
In Stylacodon, the lower jaw is long and slender, and con- 
stricted in front of the coronoid process, which slopes well up- 
ward and backward. The condyle is convex transversely, and 
placed considerably above the line of the teeth. The jaw is 
shallow below the molars, scarcely exceeding the height of the 
teeth themselves, while the lower border in this region is nearly 
straight. These differences may be readily seen in the two 
specimens. shown on Plate IX, figures 1 and 2. The mylohy- 
oid groove is well developed in both genera, and its position is 
essentially the same in each. 
In Dryolestes, the mental foramen is below the first pre- 
molar. The dental foramen is beneath the front margin of the 
coronoid process, and at this aperture, the mylohyoid groove 
begins. 
ASTHENODON. 
The genus Asthenodon, the type species of which is de- 
scribed below, agrees with the above genera in the more im- 
portant characters of the lower dentition, but differs, in having 
the entire series of teeth much more uniform i in size, and but 
eleven teeth behind the canine. The type of this species is 
the lower jaw shown in Plate IX, figure 7, A second speci- 
men referred to this species is the anterior part of another jaw, 
shown in figure 6. The former jaw shows a weak canine (a), 
followed by three premolars, each with two fangs. Behind 
these, in place of the large, trenchant premolar seen in Dryo- 
lestes and Stylacodon, is a small tooth, which from its shape 
may be regarded as the first molar. The remaining teeth agree 
in their more important characters with the corresponding 
molars of Dryolestes. The second specimen, figure 6, shows a 
similar weak canine, and, in front of it, the four incisors in 
place, increasing rapidly in size forward, the front one being 
larger than the canine. 
Asthenodon segnis, gen. et sp. nov. 
In the genus Asthenodon, the inferior dentition on each side 
is as follows: 
Incisors 4; canine 1; premolars 3; molars 8. 
The largest tooth in the entire lower series is the first incisor, 
Plate IX, “figure 6,1. The remaining incisors decrease in size 
backward, as shown in the same figure, 2,3,and 4. The canine 
(a) is small and weak, and its crown resembles that of the 
incisors. It is implanted by two roots, which are nearly con- 
nate. The three premolars behind the canine have each two 
fangs, and increase in size from first to last, as shown in figure 
7, of the same Plate. The following seven teeth, judging from 
