TBE WASATCH FAUNA. 
279 
tibia is seen in the simple and low spine and obtuse or low crest, charac¬ 
ters which especially belong to the Amhhjpoda and Creodonta, and inferen- 
tially to the Mesodonta. Fragments of tibim of Orotlierium indicate a well- 
developed tibial crest, as in other Perissodactyla. 
The tibio-tarsal articulation in all Mammalia, including those of the 
Eocene period, is much the larger, the fibulo-tarsal being insignificant. In 
the RepUlia, it is a fixed articulation, and hence a flat one ; in the Mammalia, 
it is the ankle-joint, and more or less ginglymoid. In most recent Mammals 
it forms a pulley-joint with flat sides, and in most of these its superior surface 
is grooved, generally deeply, to receive a corresponding angular rib of the 
tibia. The only existing order which exhibits a nearly flat tibial face of 
the astragalus is that of the Prohoscidia, and the flatness is here trans¬ 
verse, the antero-posterior section being slightly convex. An approach to 
this structure is seen in a few Insectivora, where the groove is shallow. In 
the Wasatch fauna, the only species which are known to possess the 
specialized or interlocking ankle-joint are the ten referred to the Perisso¬ 
dactyla. Of the remaining forty-four species, twenty-two have an ungrooved 
astragalus, that of the Amhlypoda being flat, and those of the Creodonta 
being more or less convex antero-posteriorly. Of the remainder, it is 
almost certain that the Mesodonta and Tceniodonta, fourteen species, present 
similar characters. 
The number of the digits is an important index to the relative position 
of Mammalian genera. It is well known that specialization is exhibited in 
the orders Perissodactyla, Artiodactyla, Carnivora, Insectivora, Bodentia, and 
Marsupialia by the reduction of their number. This reduction is most 
strongly marked in the first two orders named, where but one or two toes 
remain. The condition of the Wasatch Mammals in this respect is as fol¬ 
lows: The Pantodonta and Creodonta certainly, and the Insectivora, the Bo¬ 
dentia and Mesodonta, with great probability, possessed five digits on both 
pairs of feet. These groups include forty-one of the fifty-four species. Of 
the remainder, it is probable that the ten species of Perissodactyla presented 
a smaller number of toes, as four in front, and four, or probably three, 
behind, as in the case of several of the genera of the Bridger group. 
In dental characters, many points of interest have been presented in the 
