>Not all opercula reveal the direction of growth.  Although some grow
>spirally, others do so concentrically. Those of the latter type cannot be
>analyzed to infer the orientation of the animal which secreted them.
>
>Harry

Indeed; for instance, Buccinum undatum, which has a truly concentric
evenly-ovoid operc. Even the attachment scar on the inside will not
tell you the chirality of the animal/shell; it is in effect
bilaterally symmetrical (with the lane of symmetry located
transversely).

>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.

By definition concentric means centered on the MIDDLE. If a
"concentric" operc has an offset nucleus it cannot be concentric...
it is ECcentric (ie off-center, not-centered). Likewise it is
nonsense to call bivalve growthlines or ribs "concentric", because
they are NOT (call them instead commarginal).

 From memory, the labRal (outer lip) margin of an operc will only be
thickened in coiled forms; generally concentric opercs have a thin
margin.

Labium = inner lip
labrum = outer lip, hence collabral sculpture

With elongate coiled opercs with terminal nucleus (eg Volema, Vasum,
Columbarium) the curvature of the operc is due to the coiling, and
gives the chirality away... I get sick of buying sinistral Volema and
receiving it... with a DEXTRAL operc inserted...
--
Andrew Grebneff
Dunedin
New Zealand
Fossil preparator
<[log in to unmask]>
Seashell, Macintosh, VW/Toyota van nut

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