DENTAL SYSTEM 19 
one is a higher form than the other, since they both serve important 
and different purposes in the animal economy. 
As is almost always the case in nature, intermediate conditions 
between these two forms of teeth are met with. Thus some teeth, 
as the molars of the Horse, and of many Rodents, are for a time 
rootless, and have growing pulps producing very long crowns with 
parallel sides, the summits of which may be in use and beginning 
to wear away while the bases are still growing; but ultimately the 
pulp contracts, forms a neck and distinct roots, and ceases to grow. 
The canine tusks of the Musk Deer and of the Walrus have 
persistent pulps, and are open at their base until the animal is of 
advanced age, when they close, and the pulp ceases to be renewed. 
The same sometimes happens in the tusks of very old Boars. 
The simplest form of the crown of a tooth is that of a cone; 
but this may be variously modified. Thus it may be flattened, with its 
edges sharp and cutting, and pointed at the apex, as in the laterally 
compressed premolars of most Carnivora; or it may be chisel- or 
awl-shaped, with a straight truncated edge, as in the human incisors ; 
or it may be broad, with a flat or rounded upper surface. Very 
often there is a more or less prominent ridge encircling the whole or 
part of the base of the crown just above the neck, called the cingu- 
lum, which serves as a protection to the edge of the gum in masti- 
cating, and is most developed in flesh-eating and insectivorous 
animals, in which the gums are liable to be injured by splinters of 
bone or other hard fragments of their food. The form of the 
crown is frequently rendered complex by the development upon its 
surface of elevations or tubercules called cusps or cones, or by 
ridges usually transverse, but sometimes variously curved or folded. 
When the crown is broad and the ridges are greatly developed, as 
in the molars of the Elephant, Horse, and Ox (Fig. 1, V.), the inter- 
spaces between them are filled with cement, which supports them 
and makes a solid compact mass of the whole tooth. When such a 
tooth wears away at the surface by friction against the opposed 
tooth of the other jaw, the different density of the layers of 
the substances of which it is composed—enamel, dentine, and 
cement—arranged in characteristic patterns, causes them to wear 
unequally, the hard enamel ridges projecting beyond the others, 
and thus giving rise to a grinding surface of great mechanical 
advantage. 
Succession.—The dentition of all mammals consists of a definite 
set of teeth, almost always of constant and determinate number, 
form, and situation, and, with few exceptions, persisting in a 
functional condition throughout the natural term of the animal’s 
life. In many species these are the only teeth which the animal 
ever possesses, —the set which is first formed being permanent, or, if 
accidentally lost, or decaying in extreme old age, not being replaced 
