There are various size on airial round shell. Noamaly in Japan use "Sun" measurements unit to classify shells by size. 1sun=about 3.3 centimeter. There are ２sun shell(6 centimeters across)to 3, 4, 5, 6, 7, 8 sun shell. Next,10 sun=1 shaku. So 30 centimeters shell=12 inches shell called Shaku-dama in japanese. Now we call it 3-gou, 4-gou, 5-gou, 7-gou, 10-gou, severally,too. (above photo, 2.5-gou=7.5 centimeters to 10-gou=30 centimeters this side. 20-gou=60 centimeters shell in back).
table of japanese round shell
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