Over the last couple of days, some unusual shells have appeared on the
strand line on Praa Sands beach in west Cornwall, that I can't
identify from any of our shell books. 

Very superficially, they resemble common flat-sided periwinkles
(Littorina littoralis). They vary in size from 1 to 2 cm in diameter,
so generally a bit larger than periwinkles, IME. In colour, the sides
are bluish-white, while the back (keel?) has a lavender-blue or
purplish stripe along its length. The opening is quite large and
slightly flared on the underside.

As found, an obvious feature is a transparent colourless multicelled
'sac' that protrudes from the opening. With the larger shells this is
perhaps 1.5 cm long, but correspondingly smaller for the smaller
shells. Whether this feature is an egg-sac (similar to the egg-mass of
the common whelk, but smaller, more transparent and more regularly
arranged), or a float-sac, I have no idea, but most of the shells I
found had it. The shells didn't seem to have an operculum, but I might
have missed it.

The soft parts are black or very dark brown in colour, and bleed a
purple dye when damaged. But I gather that quite a number of molluscs
are sources of purple die and this was used for dying cloth in ancient
times, so that may not be much help.

This may not be much help either, but in the strand line I also found
a horse-eye bean (Mucuna sp., probably M. sloanei), suggesting that at
least some of the drift had come from tropical America. The winds over
the last two weeks have been quite strong, anywhere from southerly to
westerly, and a couple of weeks ago we had a lot of 'By-the-wind
sailors' (Velella velella) washed ashore, although nothing like as
many as in 2004. 

Has anyone any suggestions as to what the shells might be?


-- 
Chris

E-mail: christopher[dot]hogg[at]virgin[dot]net