In a long-crowned tooth the roots are usually very short, for much of the crown itself is imbedded in jaw bone, and the longer roots are not required. Growth of the tooth is usually completed after a few years; then as it is gradually worn away it is continuously moved upward by the production of new bone under the roots, which slowly fills the bottom of the socket and continues to provide the necessary support. An equally important difference between the two types of teeth, however, is to be seen in the arrangement of the enamel, the long-crowned type being provided with this durable substance on the inside of the crown instead of having a mere protective cap on the outside.
The more complicated structure was developed from the simpler form by the easy method of deepening certain depressions located between cusps at the top of the tooth. As the crown of the tooth increased its length these depressions remained tucked in, and eventually became deep pits roughly cylindrical in shape. In addition to the enamel and dentine, a third tooth substance, known as the cement, made its appearance at about this time, and we find that quantities of this new material were deposited outside the crown enamel and also inside the enamel walls of the pit, in this way producing a firmly consolidated structure otherwise weakened by deep channels and hollow pockets. The cement differs only slightly from the dentine but is deposited while the uncut tooth is in the gum tissues of the mouth, the enamel and dentine elements being formed earlier in the embryonic tooth before it emerges from the jaw bone.
A tooth constructed by such a process, if cross-sectioned through the crown, will be found to consist of successive layers of hard and softer materials. In living animals the top of the tooth soon wears off and the enamel layers stand in higher relief because of their greater resistance to wear. A roughened surface of excellent grinding quality is thus provided, and as long as the wear continues there remains the same relative amount of enamel to retain the roughness, and resist abrasion.
Among the various types of grazing animals there is a marked difference in the arrangement and form of the enamel layers. Within a species of genus, however, the complicated enamel patterns of the molar teeth are consistently similar. In the case of horses especially, these patterns provide a most helpful key to the identification of extinct forms. The general pattern, in any of the more modern horses, may be understood more readily if the wavy enamel layers be regarded as forming a set of cylinders with deeply crinkled walls. Near the outer border of the tooth, surrounded by a thin layer of cement, is the enclosing cylinder which represents the enamel cap of the old-fashioned, low-crowned tooth. Inside of this is the central mass of dentine which has been penetrated by two of the deep pits previously mentioned. The original enamel cap has been depressed into these pits, forming two inner cylinders which are filled with cement. Instead of being circular in outline, when the cap is worn through at the grinding surface these inner cylinder walls are seen to be wrinkled and folded so as to produce a most irregular pattern. However, if several teeth of the same kind of horse are compared, it will be found that the edges of these cylinders produce figures which are remarkably uniform and characteristic for that species.
American Mastodon (Mastodon americanus)
A true mastodon of the short-jawed type.
MASTODONS AND MAMMOTHS
Elephant-like mammals both living and extinct are classed together in a single order bearing the name Proboscidea. Living members of the group are the elephants, of which the large Indian and African species are best known. Among prehistoric representatives the most frequently mentioned in the popular literature of North American animals are the following:
The American Mastodon, an immigrant from Siberia which ranged over nearly all of the United States and Canada. It was principally a forest dweller, rarely found in plains regions, was abundant during the Pleistocene period and may have been known to the early American Indians;
The Woolly Mammoth, which was about nine feet tall. It ranged over British Columbia into the United States and across to the Atlantic, disappearing in late Pleistocene time;