You are not currently logged in.
Access JSTOR through your library or other institution:
If You Use a Screen ReaderThis content is available through Read Online (Free) program, which relies on page scans. Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Crystallography of Echinoid Calcite
David M. Raup
The Journal of Geology
Vol. 67, No. 6 (Nov., 1959), pp. 661-674
Published by: The University of Chicago Press
Stable URL: http://www.jstor.org/stable/30061961
Page Count: 14
You can always find the topics here!Topics: Calcite, Parallel plates, Optics, Echinoderms, Paper plates, Species, Fossils, Genitalia, Crystallography, Tangents
Were these topics helpful?See something inaccurate? Let us know!
Select the topics that are inaccurate.
Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Preview not available
Each element of an echinoderm skeleton is a single crystal of calcite. The universal stage method is used to determine orientations of crystals in coronal plates of fossil and Recent regular echinoids. Stereographic diagrams are presented which relate crystallographic orientation to the symmetry of the echinoid skeleton. Most species exhibit one of two preferred orientations and show little variation within a single individual. In one of the preferred orientations, c-axes are perpendicular to the test surface; in the other, they are tangent to the test surface and are aligned longitudinally. In the present sample, crystallographic orientation shows no variation within single genera, and most echinoid families studied have but one orientation. The two preferred orientations appear to have evolved independently several times during echinoid history. Preferred orientation, as an adaptation to the environment, may be related to the organisms' sensitivity to light.
The Journal of Geology © 1959 The University of Chicago Press