Katelysia (Nipponomarcia) nakamurai Ikebe の変異について

抄録

"Katelysia" (Nipponomarcia) nakamurai Ikebe has been well known to Japanese geologists and paleontologists as an index fossil of middle Miocene. The genus Ketelysia is not so proper for this species that the subgenus Nipponomarcia should be used as a genus. As this fossil was variable in its form, an attempt was made to determine the nature of variation. The material consists of four groups. A and B were collected from fossil localities in the Iwamura basin, Gifu Prefecture and C and D from Okuyamada, Kyoto Prefecture. The length of shell (L), the greatest dimension which ran from the apex of beak to a point of ventral margin (B), and the thickness of valve (T) were measured. The distributions of B/L and T/L are shown in figs. 1-4. Means and standard deviations were estimated and then compared with each other. From the above results, it may be concluded : 1) The difference between four groups is remarkable and especially D is different from the others. 2) A and B are smaller than C and D in form. 3) The variation of B/L is more conspicuous than that of T/L. 4) The variation with growth is not found.