340 O. C. Marsh—American Jurassic Mammals. 
Spalacotheride. 
The type genus of this family is Spalacotherium of Owen, 
but it is probable that he included more than one generic form 
under this name, in the various specimens described. In this 
country, one well-preserved jaw has been found, which appears 
to indicate a distinct genus (J/enacodon), and is described 
below. This specimen is represented on Plate X, figures 5 
and 6. 
In the typical specimens of Spalacotherium, the pre- 
molar and molar teeth are ten in number, and of the same 
general form. ‘The crown consists of one, main, external cone, 
high and pointed, and two, short, inner cusps, nearly equal in 
size, in front of and behind the main cone. The canine has 
two fangs, and there is little or no diastema behind it. 
In Menacodon, the molars have the same general form, but 
there appear to be but seven in the post-canine series. The 
crowns also are shorter and more robust. ‘The canine is small, 
and has two roots. 
Menacodon rarus, gen. et sp. nov. 
In this species, the lower jaw is comparatively slender, and 
its inferior border is strongly convex, longitudinally. The 
canine was small, and directed well forward. The first three 
premolars are separated slightly from the canine, and from each 
other. The three following teeth, which may be regarded as 
true molars, are larger and more elevated, and behind these 
was the last molar, somewhat smaller in size. 
In the type specimen of J/enacodon, there is no sharply 
defined mylohyoid groove, but a shallow depression takes its 
place, as indicated in Plate X, figure 6. 
In Spalacotherium, there is a well-defined mylohyoid groove. 
The unique specimen on which the present species is estab- 
lished was found in the Upper Jurassic of Wyoming. 
Tinodontide. 
This family is well represented by American forms, one of 
which, the type species of Z%nodon, is shown on Plate X, fig- 
ure 1. Phascolothertum, Owen, appears from its dentition to 
be an allied form, but differs in several important points, and 
may yet be found to represent a distinct family. The premo- 
lar and molar téeth have nearly the same form in both genera, 
but in Zinodon, there is a larger number of post-canine teeth. 
The coronoid process, also, is vertical, and the angle of the jaw 
is not inflected. The premolars have the same general shape 
as the molars, the crowns being composed essentially of three 
