Fig. [77] illustrates the centrals with two rudimentary cusps, the laterals with one, and the cuspids with one also. Fig. [78] represents cusps upon the lingual surface of the molars. The cuspids are not unlike the lower cuspids with a rudimentary lingual cusp.
FIG. 78.
There is a gradation from central incisors toward the bicuspids, in evolution. This grading of form is not observed in passing from the cuspid to the bicuspid in man. But the cuspid often presents a cingulum on the lingual face that inclines it toward the bicuspid forms in lower mammals, like the mole, and the first premolar, or bicuspid, is then more caniniform, the inner tubercle being much more reduced. This inner tubercle is very variable and erratic as to its position. It appears as far front as the centrals and is often present on the lingual face of the laterals of man. The lingual tubercle is very constant on the first bicuspid of man and is as well developed as the buccal. But in some lower forms, as in the lemurs, it is quite deficient. It attains the highest development only in the anthropoids and man. Considering these stages of development, the grading from the cuspid to the bicuspid forms was more gradual in the earlier species than in the later, where the individual teeth have taken on special development.
FIG. 79.
The skull of a degenerate girl who died from tuberculosis, at thirteen years, presented, among other stigmata, a cusp on the external surface of a right inferior cuspid. In Fig. [79], where every tooth is present, a most remarkable display of cusps occurs. The cusps upon the cutting and grinding edges are not obliterated. Commencing with the left superior central incisors, three cusps are present with a rudimentary palatine cusp. The laterals also show three cusps, while the cuspid has two very distinct. The first and second bicuspids have tubercular cusps, they being in line. The buccal cusps upon the molars, two or three, and are still in position. The palatine cusps are worn away. The same is the case upon the opposite side, except that the cuspid has cusps that have fused together, leaving a small projection upon the mesial side and a rudimentary palatine cusp. The cusp upon the third molar is lost. In another case (Fig. [61]) the primitive cone teeth are seen trying to shape themselves into incisors. The lateral incisors, cuspids and bicuspids are still cone-shaped. The first permanent molar is fairly formed while the second molars are still in a primitive condition.
Degenerate teeth unite in twos, threes, fours, and fives. These single, cone-shaped teeth grow together and form bicuspids and molars. The germ of any two normal teeth may intermingle and unite; not only are the crowns found united with separate roots, but crowns and roots are united throughout.