It is generally acknowledged that the deciduous dentition in men carries many more primitive characters than does the permanent dentition. For further substantiation of the theory, the present author undertook measurement on the anatomy of lower deciduous molars and made the investigation on the possible primitive traits in their coronal region. Results obtained were summarized as follows. 1. Both the mesio-distal and bucco-lingual diameters in the crown of second deciduous molar (m_2) were significantly greater than those in the first deciduous molar (m_1). In this regard, there was also noticed sex difference which was again more conspicuous in m_2 than in m_1. With regard to the size of both kinds of the teeth, there existed a highly positive correlation between m_1 and m_2. 2. As to the number of occlusal cusps occurring in both teeth, 5-cusp occupied 60.4% in m_1 and exceeded 4-cusp teeth, where as in m_2 5-cusp teeth occupied as much as 100%, suggesting that the former teeth were slower than the latter in the process of cuspal differentiation. 3. As to the patterns in which the fissure occur in the occlusal surface, Y-type was most frequent with 66.4%, followed decreasingly by Cross-type and X-type. this means that the crown pattern is consisted largely of Y5 which occupies 48.7%, followed in decreasing order by +4, Y4 and +5. 4. With respect to the occulusal pattern of m_2, those teeth that showed Dryopithecus pattern (Y5) occupied as much as 96.8% in the male and 92.3% in the female, and no other pattern was noticed to occur except a few instance of occurence of +5. 5. Those traits such as Cingulum, Mesiobuccal ridge, Trigonid incision and Distal trigonid crest could be noticed to occur fairly distinctly in m_1, but rarely noticed in m_2. These traits are considered to belong to the primitive traits of such teeth. 6. Although it has been generally believed that the occurrence of 6th cusp, 7th cusp and Protostylid in m_1 is very rare, the present study revealed the presence, although in trace amount, of each of these traits in frequency percentages of 8.6, 22.8 and 18.9 respectively. 7. On the occlusal surface patterns of m_1, the Type 2 pattern (triangle shape) occurred more frequently than Type 1 pattern (oblong shape) with as much as 69% 8. The development of the central ridge in the Metaconid of m_2 was very complete and the Deflecting wrinkle as produced by the central ridge was noticed to occur in about 2/3 of teeth examined. Although the present author presumed that these findings in m_2 had something to do with the occurrence of Distal trigonid crest in m_1, he failed to confirm any such correlationship at the statistical level. 9. The frequency of occurrence of 6th cusp in m_2 amounted to 44.8% in the male, 35.6% in the female, and that of 7th cusp amounted to 83.3% in the male and 86.3% in the female, showing rather higher occurrence of such traits in m_2 than in the molar teeth. On the other hand, there was no statistical correlation to be found between m_1 and m_2 with regard to the occurrence of 6th and 7th cusp. 10. Occurrence of Protostylid in m_2 amounted to 77.6% in the male, 70.8% in the female showing a greater frequency than that in molar teeth. No statistical correlation was noticed between m_1 and m_2 as far as the frequency of Protostylid is concerned. 11. The occurrence of Trigonid furrow in m_2 was seen in 38.0% in the male, 30.5% in the female, and of this trait more than half of whole cases were of alfa type which represents the most characteristics of primitiveness. 12. Of those physical traits described above, the Deflecting wrinkle, 6th cusp, 7th cusp and Protostylid are more frequently encountered among Mongoloid peoples including Japanese. These traits appeared most frequently in m_2, less frequently in m_1, but were rarely seen in M_2 and M_3 suggesting that these are certainly among the primitive traits of human dentition.