Dear Fabio

No problemo. I will put this on Conch-L too.

Distinguishing hyperstrophic dextral shells from orthostrophic (the normal
mode of growtn in the vast majority of gastropods) sinistral ones is a
problem; likewise distinguishing sinistral hyperstrophic from isostrophic
dextral.

The normally-hyperstrophic taxa that I can think of are: Macluritidae
(fossils from early Paleozoic), Lanistes (Ampulariidae, freshwater),
Limacina, Peracle and Planorbidae (sinistral hyperstrophic, appearing to be
dextral); larvae of Architectonicidae, Pyramidellidae, some Mathildidae,
opisthobranchs; and Ellobiidae, Trimusculidae, Siphonariidae
(Basommatophoran so-called "pulmonates").

If the shell is nonsiphonostomatous, that is, lacking a canal, it can be
difficult to decide whether it is orthostrophic (ie the usual
"downward"-coiling) or hyperstrophic ("upward"-coiling). The best way is
just to find out which are normally which, but most experts in my
experience don't know the difference between sinistral and hyperstrophic!
Or perhaps they knew, but didn't bother clarifying in their popular
publications. This included R Tucker Abbott. However I do personally know
one expert who definitely does NOT know the difference. Anyway, if the
experts don't know, how are others to find out, or even to know there's a
difference?

One way is that if there is a coiled operculum, when viewed from the inner
or attachment side the operc of a dextral shell is always coiled
sinistrally. If your shell appears sinistral but the inside of the operc is
sinistral also, then the shell is dextral and must be hyperstrophic. This
is how I worked out that my "sinistral" Turbo smaragdus is in fact dextral.
Since then I have seen 3 other specimens of this species, and heard of
others. Of course Turbo has a hint of a canal, and this can also be used...

If a siphonostomatous shell is hyperstrophic the canal will be, as usual,
at the "bottom" of the aperture, but this part of the aperture will be
pressed against the apparent "shoulder" of the penultimate whorl, as in the
Turbo. If it is a species with a long canal, the canal should be wrapped
around the penultimate whorl, and the shell will have an extremely wierd
appearance. I once drew diagrams to work out what a sinistral hyperstrophic
Busycon perversum would look like. Imagine a giant solid elongate-ovate
Cylichna, with a row of spines around the lower base! To my knowledge no
such shells with well-developed canals are known. Yet!

In nonsiphonostomatous hyperstrophic shells the shoulder is rolled toward
what in orthostrophic forms would be the apex, closing the sunken spire
off. The base is drawn upward, exposing the previous whorls and creating a
false spire. The parietal shield becomes detached from the penultimate
whorl and takes the form of an apparent columella.

Many opistobranch shells retain some degree of hyperstrohic form in their
adult shells, producing their sunken spires (to a variable degree within
species of Retusa and Tornatina). Some, such as Atys, appear to have a
columella at both ends. However the hyperstrophy does not normally reach
such a degree as to expose whorls at the base in these shells, though it
may possibly happen.

The anatomy of the animal is a dead giveaway. Certain structures (gills,
penis, any nonbilateral element) will always be on one particular side of
the body in a dextral animal, regardless of the shell's strophic condition.
With a genuine sinistral specimen the body plan is a mirror-image of a
conspecific dextral specimen.

There are more normal sinistral species than hyperstrophic dextral.
Abnormal hyperstrophy is extremely rare, and I only know it to occur in
Heliacus spp and the New Zealand Turbo smaragdus.

The thing with Peracle and Limacina is that they are basically neotenous
opisthobranchs, ie their development is arrested and they remain juvenile
in form into full adulthood. In effect they are adult veligers.

A good lay term for dextral hyperstrophy is "pseudosinistral", ie a shell
that appears to be sinistral but actually is not. For sinistral
hyperstrophy it is "pseudodextral". The alternative terms "ultrasinistral"
and "ultradextral" are effectively meaningless and are best not employed.

Hope this clarifies things and doesn't just leave you confused!

Regards
Andrew G

References suggested:
-Robertson R & Merrill A S, 1963 "Abnormal dextral hyperstrophy of
postlarval Heliacus (Gastropoda: Architectonicidae)". The Veliger 6(2):
76-79.
-Robertson R & Merrill A S, 1974, "Sinistrality unknown in the
Architectonicidae. Conchiglia 6(60): 14.
-Robertson R, 1993 "Snail Handedness". National Geographic Research &

Exploration, 9(1): 104-119.