07 October 1966
Vol 154, Issue 3745

About The Cover

The shall of the Devonian brachiopod, Pholidostrophia nacrea (Hall), has a layered, crossed, lameller structure. The calcite needles in each layer commonly indent one another and produce a surface that is an optical diffraction grating; hence the shell has a pseudonacreous luster (X approximately 710). See page 153.[K. M. Towe and C. W. Harper, Jr., Smithsonian Institution]