>You got It! Any Xenophora can only attach what is available at the time of
>growth.

Er... yes. BUT they do have individual preferences. Our common huge
Oligocene species X (X) prognata would attach shells, but if present
small flattish pebbles were far preferred. And gastropods, though
present, were almost entirely ignored.
--
Andrew Grebneff
165 Evans St, Dunedin 9001, New Zealand
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Seashell, Macintosh, VW/Toyota van nut