(1) It is understood that there are two types of acquired dental changes, intentional and non-intentional. The former is represented by the artificial removal of teeth which was frequently practiced by the Japanese especially in the later Jomon-period of prehistoric times. But very few reports have been made on the latter for which the present report is being made.
A study is made of the grooves on the surface of hard dental tissue due to mechanical wear other than ordinary mastication, i.e. abrasion specifically of buccal, occlusal and lingual surface, observed in three skeletons of the protohistoric (_??_, Mat.; _??_, Mat.; _??_, Ad.) and one skeleton of the prehistoric age (_??_, Mat.).
(2) The three protohistoric skulls show a buccal surface abrasion on the neck of maxillar and mandibular teeth in varying extent. They show a concave defect with a smooth surface (Pl. 1-4). None of them show a wedge-shaped defect, which is found in ca. 20% of the buccal surface abrasion in the recent Japanese. On experimentation, it is accepted that such defect develops when a present tooth-brush is employed on which animal hair or the like is planted.
This verifies that the abrasion and defect observed in the protohistoric skulls is not attributable to the use of a tooth-brush which is now employed.
The abrasion somtimes invades the enamel of crown. It shows a median attack starting even from the canine. It always invades premolars extensively. Distally it almost always reaches the first molar. Since abrasion is observed on the teeth in a series, it forms an almost straight, narrow and lengthy groove on the buccal surface of teeth mesiodistally. From the above findings protohistoric Japanese seems to have used a thin wood piece or bamboo piece like chopsticks for rubbing their teeth, presumbly for cleansing, resulting in a marked abrasion through an inadequate manipulation.
(3) One of the three skulls of the protohistoric age with buccal surface abrasion demonstrates additionally incisive edge abrasion of upper and lower incisors (P1. 5), as well as the abrasion of the lingual surface of tooth crown of upper incisors (P1. 6). On both upper and lower jaws, the lateral incisor demonstrates a more extensive incisive edge abrasion than the median incisor so that there is a spindle-shaped space left between the upper and lower incisors. One skull of the prehistoric age examined shows a markedly concave abrasion almost running in mesiodistal direction on the occlusal surface of lower median teeth and of bilateral distal teeth. Since the upper jaw of this skull is destroyed, it is not clear if the skull forms the spindle-shaped space mentioned above. But it seems obvious that such a marked abrasion as observed in this skull cannot be resulted from an ordinary mastication. The abnormal abrasion above may be due to the fact that these ancient individuals might have used their teeth, besides ordinary mastication, for tearing animal hides, meat and bones, as well as for tanning.