>  > Seeing these specimens side by side raises a question for me I've had for
>>  while. When I see a sinistral shell, I have a very strong sensation that
>the
>>  shell is tilted or skewed--a sensation I don't have with dextral shells.
>>
>>  This comparison makes it really obvious: at least with some sinistral
>  > shells, they really aren't just mirror images of dextral ones.
>
>They are! Hold a mirror in front of them and you'll be surprised. The
>skewing apparently is in the brain (weird enough, people who never collected
>shells do not see it!)

I noticed this optical illusion as a kid of about 9-10, holding
dextral shells up to a mirror.

>In this example of Turbinella pyrums side by side, it looked obvious to me
>that the row of knobs and suture line on the sinistral has a steep angle
>whereas the dextral is nearly horizontal.
>
>I'd known about the perceptual effect on shell collectors but I figured this
>was surely an exception.
>
>OK, I got out the mirror and held it up to the computer. Presto! the slant
>disappeared! Que fantastico! now the dextral appears to have a slant!

I suspect it is because we tend to hold a dextral gastropod shell
with the axis not vertical but tilted a bit to the right. Trying to
hold a sinistral, we do this again, and it results in the appearance
of steeper sutures. Factually, the ONLY way a suture can be steeper
is by the animal increasing the rate of translation of the generating
curve along the axis, thereby steepening the suture... this is
immediately obvious as it results in a steeper taller spire. Check a
sin/dex Busycon pair and you can see that in fact the spire-heights
are pretty much the same; the suture runs at or just below (anterior
to) the level of peripheral spines. Therefore the sutural angle must
be the same... simple geometry.

>We shell collectors see things differently. (Now, would the flesh of the
>sinistral TASTE different?)

They taste BETTER!

>The late Stephen J. Gould and collaborators*** shared David's perception
>and addressed the problem with a morphometric analysis of five anomalously
>sinistral specimens of Cerion, a Bahamian landsnail which was a already an
>established favorite of the senior scientist. Using Xray images of the
>shells to enhance the number and quality of measurements, they showed the
>sinistral shells differed from dextral mates in more than just the
>direction of coiling - the lefties had smaller apertures and a unique twist
>in the coiling axis.

Cerion appears unusual... the sinistrality occuring there may be due
to other causes than genetically-controlled reversed cell-division
from the ovum. Perhaps they are dextral hyperstrophic rather than
sinistral? (I have the various Cerion papers but can't remember, and
the papers are not accessible now).

>I've thought a lot about this matter, and I've looked at a lot of shells in
>mirrors.  On reflection (argh!), I think Steve was right for a substantial
>portion of these anomalous shells.

Which are those, Harry? Turbinidae does not appear to produce
sinistral shells, though I have seen specimens or images of at least
3 species in a dextral hyperstrophic condition. Not a Guildfordia
yoka yet, unfortunately...

--
Andrew Grebneff
Dunedin
New Zealand
Fossil preparator
<[log in to unmask]>
Seashell, Macintosh, VW/Toyota van nut

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