I have received a box of unlabelled shells that are all supposed to be
from western Mexico.  I have made IDs on most of them, in the correct
general vicinity of western Mexico except for three.  If anyone can help
me from the following descriptions I would be most grateful:
1.  A small cerith that looks exactly the same to me as the Caribbean
Cerithium litteratum as illustrated in the "Compendium", except some of
mine are up to 1.25 inches (Compendium lists 1").
2.  A small white shell that looks exactly like the Caribbean Astraea
tecta americana as illustrated in the Compendium.
3.  A bivalve that I think is a Donax, relatively strong-shelled, up to
2" long with a purple interior, exterior light brown with numerous thin
brown radiating lines.  I can't find any Donax (or anything else that
looks like it) that could be this large.  It looks similar to the
picture of Donax carinatus in the Compendium except that there is not as
sharp a point between the shorter (anterior?) edge and the ventral
edge.  Both posterior and anterior ventral corners are rounded in mine.
Could these be western Mexican shells, or were some non-Mexican shells
likely thrown in the box?  I think all of them are extremely common
considering the number of them in the box.

Nora
Calgary, Alberta