Shell not nacreous, without umbilicus, with prominent spire and polished surface.
These two layers, therefore, when once formed cannot increase in thickness; as the mantle grows in extent its border passes beyond the formed parts of the two outer layers, and the latter are covered internally by a deposit of nacreous matter.
The zone of the external surface of the mantle within the edge secretes a layer formed of prisms of calcite; the rest of the epithelium from this zone to the apex secretes the inner layer of the shell, composed of successive laminae; this is the nacreous layer, and in certain species has a commercial value as nacre or mother-of-pearl.
Boric acid crystallizes from water in white nacreous laminae belonging to the triclinic system; it is difficultly soluble in cold water, but dissolves readily in hot water.
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