[Palaeontology, Vol. 50, Part 5, 2007, pp. 1031–1037] MAGNESIUM-RICH INTRALENSAR STRUCTURES IN SCHIZOCHROAL TRILOBITE EYES by MARTIN R. LEE, CLARE TORNEY and ALAN W. OWEN Department of Geographical and Earth Sciences, University of Glasgow, Gregory Building, Lilybank Gardens, Glasgow G12 8QQ, UK; e-mail:
[email protected] Typescript received 14 March 2007; accepted in revised form 25 May 2007 Abstract: The interpretation of the lenses of schizochroal ucts reflect original differences in mineral chemistry between trilobite eyes as aplanatic doublets by Clarkson and Levi-Setti the upper lens unit and lower intralensar bowl. The turbidity over 30 years ago has been widely accepted. However, the of the bowl and of the core within the upper part of the lens means of achieving a difference in refractive index across the are the result of their greater microporosity and abundance interface between the two parts of each lens to overcome of microdolomite inclusions, both of which were products of spherical aberration has remained a matter of speculation diagenetic replacement of original magnesian calcite in these and lately it has been argued that the doublet structure itself areas. Such a difference in magnesium concentration in the is no more than a diagenetic artefact. Recent advances in original calcite has long been postulated as one of the ways technologies for imaging, chemical analysis and crystallo- by which the interface between these lens units could have graphic characterization of minerals at high spatial resolu- produced an aberration-free image and the present study tions have enabled a re-examination of the structure of provides the first direct evidence of such a chemical contrast, calcite lenses at an unprecedented level of detail.