O. C. Marsh—American Jurassic Mammals. 387 
the shape, are molars, and behind them is the alveole of one 
more. These molars agree in general form with those of Dryo- 
lestes. The form of the lower jaw also is similar in the two 
genera. The upper jaw of this genus is not known. 
The specimens representing this species indicate an animal 
about the size of a weasel. They are from the Atlantosaurus 
beds of Wyoming Territory. 
LAopon. 
A fourth genus, Zaodon, while agreeing in the general type 
of lower molar teeth with the above forms, differs widely from 
them in other respects. The molars in this genus have the outer 
main cone high and pointed, as in the above genera, but the 
inner opposite cusp is greatly reduced in size, as shown in the 
type specimen represented in Plate LX, figure 5. There ap- 
pear to have been eight molar teeth, six of which are well pre- 
served. In front of these, are two premolars of nearly equal 
size, and between these and the canine, there were apparently 
three more, each with two fangs, making thirteen teeth in the 
premolar and molar series. The canine had two roots, and the 
last incisor was placed closely in front of it. 
In this specimen, the dental foramen is situated below the 
summit of the coronoid process. Its aperture is placed ob- 
liquely, opening backward and upward, and from its outer 
margin, the deep mylohyoid groove extends forward and down- 
ward, rapidly descending below the lower border of the ramus. 
This lower jaw is intermediate in form between Dryolestes 
and Stylacodon. It has the slender straight ramus of Stylaco- 
don, with even a stronger constriction behind the molar teeth, 
but the jaw is deeper below the molar series, and the lower 
margin is convex, as in Dryolestes. The molar teeth resemble 
those of Peraspalaw, Owen, but in that genus there is a less 
number of teeth, and other features not seen in the present 
specimen. 
The upper jaw of this genus has not yet been identified. 
Laodon venustus, gen. et sp. nov. 
In the type specimen of this species, the inner side only of 
the lower jaw is shown. The alveolar border is nearly straight, 
while the lower margin is strongly convex. The anterior por- 
tion of the ramus is very shallow, but little, if any, deeper than 
the crowns of the teeth it contained. There is a well-marked 
mylohyoid groove, which begins at the dental foramen, and 
extends forward and downward, until it is lost below, directly 
under the second molar. The angle of the jaw extends well 
backward, and was not inflected, although somewhat thickened 
