Morphological Analysis of the Shovel-shaped Teeth

The shovel-shaped teeth were analysed morphologically in 560 plaster casts of upper and lower jaws obtained from Japanese in Chubu District, and results summarized as follows :(1) The incidence of shovel-shaped incisors and canines is shown in Table 1. It is somewhat lower among Japanese than among other Mongoloids, the Japanese forming a subdivision, different at least from that of Indians or Eskimos, within a division of Mongoloids.(2) A close, parallel relationship exists between the degree of development of the shovel-shaped front teeth and that of the marginal ridge on the labial and lingual surface of upper and lower canines or that of the double-shoveled incisors.(3) A close negative relationship is present between the degree of development of the shovel-shaped canines and that of their lingual central ridge. Similar relationship is also observed between the lingual central ridge of upper canines and shovel-shaped lower incisor.(4) On the basis of the preceding evidences, it is concluded that the incidence of shovel-shape is generally high in teeth bearing remarkable developed marginal ridge and slightly developed central ridge, and vice versa.(5) A parallel relationship exists between the degree of development of shovel-shaped incisors and that of lingual tubercle, but no significant relation is observed between the two in canines.(6) Mesio-distal diameter of the crown of incisors is distinctly larger in more developed shovel-shaped incisors, both upper and lower. No significant difference is observed between the two in canines.(7) Shovel-shape of front teeth is not an independent characteristics but final phenotype caused by the relative or absolute degree of development or the morphological variation of many characteristics present in the lingual surface of front teeth.