124 
Tlie Evolution of the Horse. 
unworn tooth being coated with enamel, the hardest substance 
entering into the formation of animal bodies. When the ridges 
became worn down, the dentine, or ivory of the interior, was 
exposed, forming islands surrounded by enamel. With the 
progress of time horse-like animals appeared with the crowns of 
the teeth gradually becoming longer, the valleys deeper, and 
the ridges not only more elevated, but more curved and com- 
plex in arrangement. To give support to these high ridges, 
and to save them breaking in use, the valleys and cavities 
between them became filled up to the top with a bony substance, 
called cement, and as the crown wore down an admirable grind- 
ing surface, consisting of patches and islands of the two softer 
substances, dentine and cementum, separated by variously re- 
A 3 
Fig. 2. — A. Side view of upper molar tooth of Anchitherium (brachydotit. 
form). B. Corresponding tooth of horse {hypsidont form). C. The same 
tooth of an old horse tvith the greater part of the crown icorn away, and 
the roots fully formed. D. Grinding surface of the molar tooth of a horse. 
The uncoloured portion is the dentine or ivory ; the. shaded part the 
cementum, Jilting the cavities and surrounding the exterior ; the black line 
separating these two is the enamel or hardest constituent of the tooth. 
duplicated and contorted projecting lines of intensely hard enamel, 
resulted. The crown continued lengthening throughout the 
Pliocene time, until, in the modern horse, it assumed the form 
called "hypsidont," or high-toothed. Instead of contracting 
into a neck, and forming roots, its sides continue parallel for a 
considerable depth in the socket, and as the surface wears away 
the whole tooth slowly pushes up, and maintains the grinding 
edge constantly at the same level above the bone, much as in 
the perpetually-growing teeth of rats and beavers. But in. 
existing horses there is a limit to the growth of the molar. 
After a length ia attained which, in normal conditions, supplies 
sufficient grinding-surface for the lifetime of the animal, a neck 
and roots are formed, and the tooth is reduced to the condition 
