Science 1969 Nov 28;1663909:1150-2. doi: 10.1126/science.166.3909.1150.

Crystal orientation and plate structure in echinoid skeletal units.

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The submicroscopic morphology of magnesian calcite skeletal units of echinoids, revealed by scanning electron microscopy, was compared with crystal orientation data obtained by x-ray methods and with macroscopic morphology. The Perischoechinoidea and the Euechinoidea differ with regard to the shapes of their trabeculae. Nearly all plates and spines are single crystals. A variety of diJfferent directional relations of c- and a-axes to the main morphological directions are found for different species; adjacent plates with identical c-axis orientation differ strongly in orientation of their a-axes. Fracture surfaces of single trabeculae show cleavage planes and zonal layers attributed to changes in secretion conditions.

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