神奈川県真鶴岬産の火山岩に穿孔する二枚貝類について

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Rock boring bivalves have been known to bore into rather soft sedimentary rocks such as sandstone, mudstone or shale, but not into volcanic rocks. However, the senior writer (MASUDA, 1968) lately reported on the occurrence of sandpipes due to Pholadidea sp., a boring bivalve, in volcanic rocks of Miocene age and also in the huge andesite boulders of geological formations. At that time he presupposed that the Recent Pholadidea might also bore into igneous rocks. The writers recently discovered two kinds of burrows penetrating andesite lava exposed in the vicinity of Cape Manazuru, Kanagawa Prefecture in central Japan. These burrows are of the boring bivalves Pholadidea sp., a very close to Pholadidea kamakurensis (YOKOYAMA), and Lithophaga curta (LISCHKE). The burrows due to Pholadidea sp. are usually found both on the surface of andesite lava and on the huge andesite boulders exposed between Cape Manazuru and Mitsuishi. Sometimes the burrows are found both on the upper surface and on the basal part of small boulders. Of the nearly perfectly preserved specimens, the bore holes of Pholadidea represent fig-like in shape. The inner surface of the bore holes is usually smooth but sometimes very faint impressions are present. The uppermost parts of the holes are subcircular in cross-section and gradually tend to expand towards the lower end. The profile of the holes near the base is subcircular. The longitudinal axis of the burrows is usually twisted. Compared with the shells of Pholadidea in rather soft sedimentary rocks the shells in the andesite are usually thicker and shorter in length, and the boundary between the anterior and posterior parts is more distinct and abrupt. The surface of the shells in sedimentary rocks shows imbricated sculpture due to the development of radial ribs and distinct concentric lines, whereas the shells in andesite are rather flat and rounded because of the smooth radial ribs and faint, fine concentric lines. The burrows due to Lithophaga are found on the upper part of the andesite lava exposed at Banba-ura and its vicinity, southwest of Cape Manazuru. They are usually arranged parallel with the cracks or flow textures of the andesite lava. The morphological characters of the shells and burrows are nearly similar to those found in sedimentary rocks. The occurrence and characters of the burrows of Pholadidea and Lithophaga in volcanic rocks may serve to analyse the paleoenviromental conditions at the time of deposition of the fossil boring shells bearing formations.
日本貝類学会の論文
1969-09-30

神奈川県真鶴岬産の火山岩に穿孔する二枚貝類について

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