During examination of thin sections of middle Jurassic limestones from western Scotland, a strongly pleochroic brown calcite was noticed. The pleochroism was found to be caused by scattering of the E-ray by inclusions that originate in the conchiolin of pelecypod shells, originally aragonite and from which the limestone has formed by recrystallization. Aragonite shells that are not recrystallized show the same pseudo-pleochroism but with reversed sign. Calcite similar to that described also occurs in the Purbeck beds, the Wealden, and the Forest Marble. A section is devoted to a brief description of conchiolin which consists of various proteins, and it is noted that the conchiolin layers of a shell can persist in some form in recrystallized shells.