The form of the teeth demands consideration next in order of importance. Sometimes we encounter cases of teeth that are all nearly alike in form; they have lost that morphological differentiation which already existed in the anthropoid apes; there is an insensible transition from the incisors, all exactly equal in form and dimensions, to the premolars, which also present the same appearance, passing over a tooth which it would be difficult to define either as incisor or premolar (the canine tooth). Usually in such uniform dentition there are slight diastemata.
This condition, however, is not frequently met with; it is much more usual to find this anomaly occurring only in part; the incisor teeth are all equal, or else the canine resembles an incisor or a premolar. In combination with this characteristic, it often happens that there is a diastema next to the canine.
In regard to size, the teeth may be too large, macrodontia, or too small, microdontia.
Microdontia may be due to a true and actual arrest of development of the teeth (white teeth, small and narrow, often all very much alike), or to a kind of corrosion of the teeth due to congenital dystrophism (syphilis). In this case the teeth are ground down and worn away either horizontally or laterally (filiform teeth), or again the cutting edge of the tooth is not horizontal in the two upper canines, but oblique, so that the teeth have the appearance of being broken.
Often the teeth are furrowed transversely with yellow streaks corresponding to a lack of development of the enamel.
Finally, the teeth may present various anomalies of position, which may be grouped under three heads:
a. Narrow teeth, so placed as to leave slight intervals between them.
b. Isolated teeth, planted outside the common line, or else transversely instead of horizontally.
c. The dentition does not follow the regular curved line, but shows various sinuosities, usually bending in at the point corresponding to the canine tooth.