THE EVOLUTION OF THE MAMMALIA. 235 
of the feet, however, made clear its true position. The teeth are 
the most primitive of any known artiodactyl dentition, and the 
cusps of the grinding teeth are conical in form, not having begun 
to assume the crescentic shape, which is characteristic of all 
the ruminants, in the broadest sense of that term. While not 
yet positively certain, it is extremely probable that this little ani- 
mal had five toes on each foot and the bones of the forearm, as 
also those of the leg, were separate. In the Bridger representa- 
tive of the series {Homacodon), a slight increase in stature is 
observable; the grinding teeth have grown more complex, and 
their cusps are rather pyramidal than conical, the first step 
toward the assumption of the crescentic shape ; the first, or inner 
toe, on each foot has been suppressed, making four, which are 
arranged in two symmetrical pairs, the third and fourth of 
the original five forming a larger median pair, with a somewhat 
smaller lateral pair, the second and fifth. 
In the Uinta genus {Protylopus) there can no longer be any 
question as to the camel-like nature of the animal ; its whole 
appearance being that of a miniature llama. It is still very 
small, though somewhat larger than its predecessor of the Bridger. 
The molar teeth are now completely transformed, and are com- 
posed of four crescentic cusps, arranged in two transverse pairs, 
and the dental series is complete, without any gaps in it. The 
skull is very llama-like, but has a proportionately smaller brain 
capacity, and the orbits for the eyes are still but partially encircled 
by bone. The neck has become elongate, as have also the 
limbs and feet, especially the hinder ones. In the forearm the 
ulna, and in the leg the fibula, are very much reduced, and have 
begun to coalesce with the enlarged radius and tibia respectively. 
There are still four digits in the fore foot, though the lateral pair 
has become much smaller than the median pair, while in the 
hind foot the lateral toes are reduced to mere threads of bone, 
which can have had no functional importance whatever. 
The next step is the White River genus (Poebr other ium) which 
marks a notable advance, and even within the limits of the 
White River beds, the successive species of the genus continue 
