Hi Gunasena,
From the Compendium: "Hoof-shells--Families Hipponicidae and Capulidae; The
shells of hoof (Hipponicidae) and cap (Capulidae) shells resemble limpets,
but the animals are prosobranchs with one set of gills and 7 rows of teeth.
The shells usually have an outer rough 'skin' or periostracum. Eggs are
brooded under the female's shell."

I was thinking about it a little more and realized that as a snail (or
hermit crab) usually is usually positioned, the darkest spot would probably
be near the aperture. So our "hoof-shells" or Sabia might migrate to a spot
near the aperture in order to retreat from the light as well as to locate
nearest the attractive chemical odor emanating from the snail (or hermit
crab?). This still doesn't explain the regularity of its position in the
parietal region; it could also "choose" to locate above the aperture. Well,
this is all conjecture on my part. But it may be a multi-factorial process
for these critters to find the "low-post position" (to borrow a phrase from
basketball).

David

>Hello David Kirsh,
>Could you please discribe bit more on Hzpponix, dose this has any connection to
>UMBONIUM family. I have seen some thing close to what you discribe but I
>don't have
>any specimens with me.
>Gunasena de silva
>Sri Lanka - e-mail <[log in to unmask]>
>
>
>
>At 12:04 PM 1/17/00 -0500, you wrote:
>>Folks,
>>Does anyone know the name of the ubiquitous cap-shell (Hipponix? or
>>whatever the newer name is) that invariably attaches singly or in pairs to
>>the parietal area of shallow water gastropods in Sri Lanka?
>>
>>And how does it "know" how to find the parietal area?
>>
>>It's about 5mm in diameter, white, with radiating ribs. Apex is elevated
>>and pointed to one side.
>>
>>David Kirsh
>>Durham, NC
>>