>Do you know why Cucullaea have this strange lamella inside each valve ?

It's a characteristic of several arcoid families... Extinct
cucullaeids eg Latiarca also show this, as do Limopsis (both genera
abundant here in the Oligocene-Pliocene). It's part of the adductor
scar structure, but I don't know for sure why it's built-up like
that. However in extinct Latiarca the scars are much more massive and
so is the entire shell, which can be over 6mm thick; it may be built
out in Cucullaea in order to increase attachment area and shorten the
muscle (increasing mechanical advantage) while not increasing shell
mass (which makes the entire animal heavier, alters its center of
gravity and also takes more energy to produce, as Geerat Vermeij
would say).

Fossil Latiarca tend to be very large (100mm plus), greatly inflated,
thick and heavy, with massive adductor buttresses, large valve
overlap (asymmetry) and a ligament so heavy and calcified that it is
commonly still present 25 million years later (not very elastic,
though!).
--
Andrew Grebneff
165 Evans St, Dunedin 9001, New Zealand
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