334 O. C. Marsh—American Jurassic Mammats. 
present specimen that its form and dimensions are uncertain. 
The fourth premolar is very large, notched at the summit, and 
with its outer face showing distinct marks of wear. 
The first true molar is small, and its crown much inclined 
backward. The second true molar is wanting, but its alveoli 
show that it was also small, and placed below the first molar. 
In this species, the series of four lower premolars is placed 
on a curve, and acts as a single cutting blade against the com- 
pressed upper premolars. This curve is completed behind by 
the two molars, which have their crowns inclined outward. 
The second and larger lower jaw referred to this species 
gives some additional characters. The third and fourth pre- 
molars show distinct traces of wear on their outer surfaces. 
The first true molar is placed obliquely, as in the previous 
specimen, and has been subjected to much attrition. The last 
true molar was situated lower than the first, and was also 
oblique. In Ctenacodon serratus, the two lower molars are 
nearly on a level. The present specimen shows that the angle 
of the jaw was strongly inflected, and there was likewise a 
ridge on the opposite outer margin. The coronoid process had 
its front border more nearly perpendicular than in Ctenacodon 
serratus. ‘There is no trace of a mylohyoid groove. 
The known specimens of this species are from the Upper 
Jurassic deposits of Wyoming. 
Dryolestide. 
The first American mammal found in the Jurassic, and a 
large proportion of those since discovered, belong to a peculiar 
family which the writer has called the Dryolestide. It in- 
cludes several genera and numerous species from this country, 
and is likewise represented among the forms found in Europe. 
The type species of the group, Dryolestes priscus, was based 
upon a characteristic lower jaw, although the specimen was 
imperfect. A nearly complete lower jaw of the species is rep- 
resented on Plate IX, figure 2. An allied species, Dryolestes 
vorax, is shown on the same Plate, by figures 3 and 4. Styla- 
codon, Asthenodon, and Laodon, other genera of this family, 
are likewise represented on the same Plate. 
The upper jaws of several genera of this family are now 
known with tolerable certainty, and these will be figured and 
described fully in the Memoir now in preparation. 
All the genera of this family have more than the typical 
number of teeth (44), and the general characters of the inferior 
dentition are well shown in Plate IX. The lower teeth form 
a close-set series, without diastemas, or marked interspaces. 
There are three, or four, incisors, and in those preserved, each 
has a distinct crown, and the series diminishes in size from in 
