I believe that most opercula with a concentric pattern of growth could still
be identified as sinistral vs. dextral. The labial edge of most opercs can
be identified by the thickened callus on the underside. The deposition of
opercular material around the nucleus is often not uniform, resulting in an
offset or eccentric nucleus. In some species/families the nucleus is close
to the columella, in others close to the anterior or posterior end of the
operc. And, the overall shape of a concentric operculum tends to be defined
by the shape of the shell aperture, being broader at one end and narrower at
the other, or angular on the columellar side and smoothly rounded on the
labial side. A concentric operculum which is perfectly oval in outline and
has a central nucleus would be difficult to classify, but such opercs are
the exception rather than the rule; but even in such an operculum the
anterior-posterior orientation could probably be determined by careful
examination of the labial callus, by someone familiar with such things.

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