Experimentally induced shell repair in the land snail Otala lactea has been employed to study the influence of the substratum on microtopography of the surface of mineral deposited by molluscs. Substrata, including the periostraca of four species of molluscs, surface replicas, and the outer membrane of the eggshell of the hen, have been inserted in the area of shell repair in Otala, and the topographical patterns of deposition of calcium carbonate have been observed by scanning electron microscopy. The mineral patterns formed on all inserted substrata conformed to the microtopography of the surfaces and were distinctly different from that of normal shell regeneration. The topography of the normal outer mineral surface of the shell of four species of molluscs were observed to conform to the topography of the periostraca on which the crystalline shell is deposited. It is inferred that the topography of the outer mineral surface of the shell probably results from the interaction of crystals of aragonite and calcite growing in an organic matrix and in contact with the periostracum. Normal shell of Otala consists of fine-grained aragonite in highly preferred orientation. Regenerated shell contains calcite grains in random orientation and aragonite, usually in random orientation. The aragonite-to-calcite ratio varies widely and appears to be independent of experimental substrata placed in the area of repair.
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