The aragonitic nacreous layer (mother-of-pearl) of Holocene pelecypods and gastropods has been studied by scanning electron microscopy of inner shell-growth surfaces. Three distinct patterns of crystal formation and growth have been distinguished which are taxonomically significant.

The pattern developed in the nacre of the Atlantic pearl oyster, Pinctada radiata Leach, is characteristic of pelecypods. Thin broad mineral laminae overlap at the growth surface giving it the appearance of a series of long, parallel steps. Growth occurs along the free margins of individual laminae through the deposition of discrete, tabular crystals which, by lateral expansion, coalesce and merge evenly with the laminae.

In the family Pinnidae, crystals are deposited in long, parallel rows spaced 15-20 µ apart. Euhedral crystals observed in Pinna carnea Gmelin are commonly eight sided; their long dimensions parallel the rows. With lateral expansion, the crystals merge end-to-end to form strips over which new crystals are deposited. At depth, adjacent strips merge to form the broad laminae which characterize the nacre.

In the gastropods Cittarium pica (Linne) and Astraea caelata (Gmelin), no stepped patterns or crystal rows are observed. Instead, crystals are deposited in tall vertical stacks spaced 5-10 µ apart. Crystal anlages deposited at the tops of the stacks expand laterally as deposition continues and eventually merge at depth with crystals of adjacent stacks to form continuous laminae. In A. caelata crystals are euhedral and commonly six sided. Crystal orientation is constant within stacks, but not between stacks.

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