The frequency and morphology of mandibular tori and the correlations between their development and environmental factors (number of teeth, degree of crowding, degree of dental attrition) were examined in plaster casts of present-day Japanese dental patients and students. A total of 224 patients (118 males aged 13-77, average age 50; 106 females aged 15-81, average age 49) and 113 students (60 males aged 17-36, average age 20; 53 females aged 18-29, average age 19) were included in the examinations. Before the examinations, we defined a new torus grading system with four categories. There were no statistically significant differences between males and females and between the left and right sides in the distribution of torus classes and positions. Palpable tori were found in 76.6% of patients and 72.0% of students. Perceptible tori were found in 70.3% of patients and 58.0% of students. Tori were most frequently found below the first and second premolars. In students, the torus class and size negatively correlated with the degree of crowding and positively correlated with the degree of dental attrition. In patients, torus class and size positively correlated with the degree of dental attrition, the number of teeth, and age. From these results, we suggest that mandibular tori are promoted by masticatory stress and other factors correlated with age.

Although four-rooted mandibular first molars have been observed on a few occasions in the literature, to the best of our knowledge, four-rooted second molars have not been reported. Here, we describe a mandibular second molar with four roots and canals: two mesial and two distal. Mesial roots were separated in the cervical one-third near the cervical margin, while distal roots were divided at a lower level in the middle one-third of the root. We discuss the possibility of different root and canal variations of the mandibular second molar from a developmental point of view.

The frequency and morphology of mandibular tori and the correlations between their development and environmental factors (number of teeth, degree of deviation of dental position, and degree of dental attrition) were examined in 105 Jomon people, 90 medieval Japanese and 105 modern Japanese. The palpable tori were found in 83.5% of Jomon people, 62.3% of medieval Japanese and 64.8% of modern Japanese. The perceptible tori were found in 70.1% of Jomon people, 18.2% of medieval Japanese and 22.0% of modern Japanese. The *日本大学松戸歯学部解剖人類形態学講座 〒 271-8587 千葉県松戸市栄町西 2 丁目 870 番 1