抄録
Writers found many holes made by boring action of Parapholas quadrizonata on the shell surface of Crassostrea gigas, which built up a reef in situ in the upper part of the Kamiiwahashi formation in the environs of Yato-machi, Chiba City. The reef is 2 m in the thickest part and is spread horizontally in the distance of 50 m about. And it is composed of a single species, which is Crassostera gigas (THUNBERG). At the higher horizon than one-third of thickness of that reef, they could find many shells of Crassostrea gigas having holes made by Parapholas quadrizonata. Number of holes found on a single specimen of the former becomes larger as it ascends to the upper horizon of the reef, however the mean value of their number is about five to a sigle specimen. In the middle part of the reef, they found Trapezium (Neotrapezium) liratum (REEVE), Batilaria zonalis (BRUGUIERE) and B. multiformis (LISCHKE), which were buried in the silt cementing shells of Crasssostrea gigas. And in the upper part of the reef, they found Anadara (Tegilarcd) granosa (LINNAEUS), Cyclina orientalis (SOWERBY) and Saxidomus purpuratus (SOWERBY), all of which have two valves closed. Considering from writers' observation about the occurrence of the above-cited molluscs, they discussed paleoecology of Parapholas quadrizonata, which had been living in the tidal zone of the depth of the Paleo-Tokyo Bay during the deposition of the upper part of the Kamiiwahashi formation as follows. The larvae of Parapholas quadrizonata were scattered in the sea water, and some of them attached to the shell surface of Crassostrea gigas at the time when the latter had grew farely larger. As the former didn't drill through to the inner surface of the latter, and the openning of their holes often toward to the free side of the openning action of two valves of the latter, they assumed that these two species had the relation of mutualism. At the later stage of the growth of the reef, muddy sediments piled on this reef within short times and stopped the more development of the reef.
