The photo below shows a section of a small piece of Cambrian material from Kinnekulle, Sweden which I recently acquired. The material is, I believe, anthroconite or orsten (limestone) found in lenses in Alum Shale. The disarticulated trilobite shields that populate the surface are from the tiny Agnostus pisiformis. My question is this: What is the very thin white layer that covers parts of the trilobites - both the pieces where the dorsal/convex side of the shell is exposed and where the ventral/concave side of the shell is exposed? Any ideas would be much appreciated.