Study of the shell structure of Pholidostrophia (Mesopholidostrophia) nitens Williams from the Silurian of Gotland using the electron microscope indicates that the nacreous luster commonly exhibited by this species is due primarily to the presence at the shell surface of a set of natural optical diffraction gratings made of calcite, as in middle Devonian P. (Pholidostrophia). A large collection of P. (Mesopholidostrophia) nitens typically exhibits intergrades between individuals with a nacreous luster and associated well developed diffraction grating structure and individuals with a dull luster and associated shell structure in which the diffraction grating pattern is erratically developed or absent; the shell structure associated with a nacreous luster and that associated with a dull luster may co-exist on the same shell. This variation is attributed either to intraspecific variation in primary shell structure, or to the diagenetic alteration of the primary shell structure of individuals with a dull luster. A single individual may exhibit in different parts of its shell the shell structures associated with both a nacreous luster and a dull luster. This is considered further evidence for regarding the species Lissostrophia (Mesolissostrophia) pellucida Williams and P. (Mesopholidostrophia) nitens as conspecific.