抄録
Both ancient and modern borings are of interest to the paleontologist. Numerous clasts derived from the Late Miocene volcaniclastic sediments, mainly rhyolite, are washed ashore on Hamanaka Beach, Yoichi Town, Hokkaido. The rhyolite of the Kucchan Formation are indicated 6.7 Ma by K-Ar dating. Bored rhyolite cobbles including bivalve shells are collected on the stranded line, and recognized as the first record of rhyolite boring in Japan. The host rock are typical glassy rhyolite, with phenocrysts of quartz, plagioclase, biotite and apatite. The borings were produced by the rock-boring clam Penitella kamakurensis. The Clavate borings are assignable to Gastrochaenolites isp. due to incomplete for accurate identification.
https://ror.org/02gn38d51 
