>1. probably Chiton olivaceus (Spengler, 1797) At 10 mm it would be a
juvenile.

It is not this species, neither as a juvenile.
I hope some mediterranean chitons expert shows up otherwise I will try to
make a tentative identification on my own (but I do not understand much of
them).

>4. Cerithium vulgatum (Bruguiere, 1792), slightly immature. At 51 mm,
>the only other species it could be is C. alucaster (Brocchi, 1814) but
>the markings and sculpture of your specimen, in my opinion, suggest C.
vulgatum.

It is probably Cerithium haustellum Monterosato in Crema, 1903.
It differs from C. vulgatum by having the last whorl small compared to
shell height.
In C. vulgatum, the last whorl can be one third of the shell height.

>6. probably Diodora graeca (L., 1758)

I do not think it is D. graeca.  It has much thicker radial and
concentrical ribs.
Another difficult group.

Best regards,
Paolo

Paolo Giulio ALBANO
Bologna, ITALY
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Homepage: http://members.xoom.virgilio.it/pgalbano/index.htm