THE EVOLUTION OF THE MAMMALIA. 45 
the grinding teeth. These are beginning to elongate and grow 
more before forming their roots, though this elongation is far less 
than it became in subsequent genera ; at the same time the val- 
leys and irregularities of the crown are covered with a thin 
deposit of true bone, which is called " cement," when developed 
upon the teeth. The limbs and feet are somewhat heavier and 
stouter than in Mesohippus, but otherwise there is little change. 
Desmatippus leads into the characteristic Loup Fork genus, 
Protohippus, which carries the advance one stage further. In this 
genus, which is larger than its predecessor, the whole appearance 
of the skeleton is so like that of the horse, as to require a careful 
examination to note the differences. All the teeth have become 
much higher crowned, though still lower than in existing horses, 
and the grinding surfaces, while very complex, are somewhat 
simpler than in the latter. This change in the character of the 
teeth, which is repeated in many other groups of mammals, has 
been explained by the great spread of the grasses, which occurred 
at this period. Grasses contain much flint and rapidly abrade the 
teeth, and this abrasion is compensated by the persistent growth of 
the teeth for a longer time. To return to Protohippus, the skull 
is thoroughly equine in appearance, and the orbit is completely 
encircled in bone ; the neck is elongated and heavy, and the limbs 
have nearly the same proportions as in the horses. The feet, 
however, still have three toes each, though the lateral digits are 
now so reduced that they scarcely touch the ground, and must 
have been mere dew-claws. In stature Protohippus about equals 
a pony or donkey and is thus larger than most of its predecessors 
and smaller than most of its successors. 
The change from Protohippus to the existing genus Equus is a 
comparatively small one, and occurred in the early Pliocene. An 
increase in height and weight of body, in the length of the teeth 
and in the complexity of their grinding surfaces are features of 
this change, but most marked is the nearly complete reduction of 
the lateral digits, the rudiments of which are now entirely con- 
cealed beneath the skin, and by horsemen are called " splints." 
The whole weight of the body is now supported by the enlarged 
