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pearly trochidae
08-14-2011, 06:25 AM,
pearly trochidae
Hello folks,

Thanks for bearing with a total novice here. My daughter found this top shell on the beach on the Pacific coast of Costa Rica. From the little poking around I've done, I'm guessing it was a less-interesting looking top shell that has been worn away to its mother-of-pearl interior. I can even see some remnants of white exterior around the edges of the largest whorl. But the thing I can't find is an example with the beautiful, bright teal color, which is clear at the tip and at the lines between the whorls. It's as green as an emerald.

The picture is here, and you can click on it to enlarge it:
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Any ideas?

thanks for your help.

08-14-2011, 09:02 AM,
Re: pearly trochidae
Hi Debbie,

You are certainly correct in your assessment of what family this shell belongs to, and what happened to it.  There are more than 50 species of Trochidae found in the area where your shell was found.  I'm pretty sure your shell is a member of the Tegula group because it appears to be a fairly solid medium-size shell and also fairly tall for its width.  Most of the other trochid groups in that area are quite thin-walled, relatively fragile shells that wouldn't be able to take this degree of erosion without being completely destroyed - and/or they are lower in form, wider than they are tall - and/or they are very small species, less than 1 cm in width.  So, assuming it is a member of the Tegula group, that narrows it to about 20 species.

Your shell has three additional characteristics that are helpful.  It has some evidence of spiral threads (fine ribs) on the surface of the whorls, though they are reduced by erosion; it has moderately strong spiral ribs on the base, stronger near the center than near the periphery (some species are smooth on the base or have only very fine ribs); and it has a closed umbilicus.  The umbilicus is an opening located centrally on the base, and extending up through the central axis of the shell.  In some species the umbilicus is open, so you can look into it and see right up to the apex of the shell.  In other species, like yours, the umbilicus is closed.  I checked these characteristics against the various Tegula species in the region.  I could come up with only one species that seems to match all these characteristics.  In fact, to my surprise, it is the only Tegula in the region with a closed umbilicus!  That species is the "snakeskin top shell", Tegula pellisserpentis.  In life the outer layer of the shell is brown, with blackish markings supposedly resembling the coloration of some types of snakes.  Also, in examining my specimens of this species, there is a greenish tint showing around the aperture of some of them, and on the apex of a couple of them.  This species reaches almost 2 inches in height, and your specimen is considerably smaller, but it may simply be a young specimen, not yet fully grown.  They all start out life small. My adult specimens range from 1 to 1.5 inches.  I still can't guarantee this ID, but its my best guess based on the available evidence.  Here are a couple of pictures of adult specimens of this species:

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<!-- m --><a class="postlink" href=""> ... ta-ban.jpg</a><!-- m -->

If you would like one of these for comparison, email me and I'll be glad to send you one.
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08-15-2011, 01:47 PM,
Re: pearly trochidae
Hi, me again ...  I ran a little experiment.  Though the evidence seemed to point to Tegula pellisserpentis, I wasn't completely satisfied with the idea of a large heavy brown and black shell being eroded into a smooth pearly shell with bright green highlights.  So, I took a small T. pellisserpentis (22 mm), and dropped it into vinegar for a few hours.  The vinegar, being acidic, erodes away the shell chemically, simulating the way rolling around in the sand erodes a shell physically.  The result was pretty interesting:

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