Abstract
A strain of microphthalmic mice was established from progeny of the X-ray irradiated DD-strain of mice (KASHIWABARA et. al., 1957, 1958) . In this strain, Irregular occlusion of incisors has been occured frequently.
Detailed morphological observation on skull revealed that Fusions of frontal and parietal bones, Bent of fronto-parietal bones, and Bent of nasal bones, occured as frequently as the occurence of Irregular occlusion of incisors. The percentage in the occurences of Fusions of frontal and parietal bones was 77.5%; Bent of f ronto-parietal bones, 32.6%; and Bent of nasal bones, 41.9%. The percentage in the occurences of Irregular occlusion of incisors was 14.7% (Table 1) .
The correlation between the asymmetrical Fusions of frontal and parietal bones and the occurences of Bent of the fronto-parietal bones was very high. The correlation coefficient was 0.892±0.062 (Table 2) . There was no correlation between occurence of Bent of the fronto-parietal bones and Bent of the nasal bones (Table 3) . It was observed that as the direction of Bent of fronto-parietal bones and Bent of nasal bones coincided, the occurence and manifestation of Irregular occlusion of incisores increased (Table 4) .
These facts would suggest that Irregular occlusion of incisores would be caused by Bent of frontoparietal bones and Bent of nasal bones. Furthermore, it would be indicative that Bent of f ronto-parietal bones would be caused by the occurence of asymmetrical Fusions of frontal and parietal bones.
