Nora,

No different ideas...all variations in expression of the same one, that
the high shell has a cooling function...achieved by greater shell
surface... and that shells higher up the shoreline are exposed to more
sun and so need greater cooling abilities; the increased cooling ability
is apparently more important to the sun-challenged animal than would be
the protection from predation and wave scouring wave action that shell
with a lower profile might afford.

References:

Yonge and Thompson, 1976. Living Marine Molluscs.

Freek Titselaar's introduction to his Revision of
the recent European Patellidae in Vita Marina 45(3-4): 21-62.

Vermeij,G. 1993. A Natural History of Shells.

Thnaks for your own contribution.

Lynn Scheu
Louisville KY
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