859. FUNCTIONAL AND TAXONOMIC IMPLICATIONS OF INTERNAL RIBS OF PROPEAMUSSIUM

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  • 859. Propeamussium の内肋の機能的および分類学的意味
  • Propeamussiumの内肋の機能的および分類学的意味〔英文〕
  • Propeamussium ノ ナイロク ノ キノウテキ オヨビ ブンルイガク

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The macroscopic and microscopic features of internal ribs in several living and fossil pectinaceans from the western Pacific were observed, and their functional and taxonomic significances were considered. Propeamussium shares internal ribs with Amusium and some other genera of the Pectinidae, but the resemblance must be superficial because their fundamental structure is quite different. The commonly paired internal ribs of pectinids are formed by the thick secretion of outer foliated layer near the ventral margin and primarily serve the tight interlocking of valve margins. On the other hand, each of the internal ribs of propeamussiids is represented by a lecticular core of fibrous prismatic (or foliated) calcite almost imbedded in the inner layer of crossed lamellar aragonite. Genetically, the cores seem to have originated from the "middle layer" (a thin layer beneath the prismatic layer) in the right valve and from the outer layer in the left valve and to be produced by some patchy isolation of shell-secreting mantle tissue. The distal ends of internal ribs are just apposed to those of the counter valve so as to contact each other when the valves are tightly closed. The internal ribs of propeamussiids may be functionally analogous to the auricular crura of free-living pectinids ; the distal ends probably serve as buttresses to counteract not only external compressive stresss but also powerful contracting force of quick adductor muscle for swimming.

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