278 
THE WASATCH FAUNA. ' 
liLimerus are divided into two portions by a rib, which is nearer the external 
than the internal margin. One of these surfaces articulates with the ulna; 
the inner portion, or trochlea, chiefly with the radius. In the Mammals with 
the least specialized limbs, this rib, which is called the intertrochlear rib, 
is wanting, as in the reptiles. Another character of the Mammalian humerus 
is the flange with acute border, in which the condyles terminate' at their 
.inner extremity, and which is present in nearly all types, but differs greatly 
in the degree of its prominence. The intertrochlear ridge is present in Artio- 
dactjjla, Perissodactyla* higher Quadrumana, and a few of the Carnivora. In 
the Bears and Cats, it is wanting, as also in the Prohoscidia and in the lower 
Mammalia generally. If now we consider the species of the Wasatch beds, 
we observe that this character is present in the ten species referred to the 
Perissodactyla, while it is absent in the forty-four remaining species. This 
conclusion is based on observation in the case of the Amblypoda, Mesodonta, 
Rodentia, Tceniodonta, and Creodonta. 
The differences in the cubito-carpal articulation are seen in the relative 
shares taken in it by the ulna and radius. In the lower terrestrial Verte¬ 
brates, these shares are more nearly equal than in the higher, and some 
range in this respect is observable in the types of the MammaUa. In the 
Wasatch genera, it has only been observed in the Amblypoda, Creodonta, 
and Perissodactyla, and the radio-carpal articulation found to be relatively 
wider in the last and highest order than in the two extinct groups first 
named. In the Amblypoda, the radial articulation is as large as the ulnar. 
As regards the fixed articulation between the ulna and radius, the 
reduction of the former bone seen in the higher Artiodactyla and Perissodactyla 
is unknown in this fauna, and even the Wasatch Perissodactyla (Orotherium) 
do not display the interlocking articulation between the .two, seen in the 
humeral cotylus, in nearly all the representatives of the two above-named 
orders of the later and present periods. 
In the less specialized forms of femora, the distal ends are not distinctly 
divided into two condyles and a rotular surface, the three being more or 
less continuous. Such femora are not found among the genera of the fauna 
under consideration. Unspecialized character in the proximal end of the 
*Not well marked in Rhinoceridce. 
