a. is Chamalea gallina
, a Venus Shell and c. is Spisula subtruncata
. I'm not sure about b. though. It is sometimes helpful if you can see the inside of the shell with bivalves as the position + shape of things like the adductor muscle scars ( the two big marks on either end of the shell ), the pallial line and sinus ( the line that joins the two adductor muscle scars and the indent in the line ) and the teeth by the hinge as they can be specific to genera.
It's just come to me
. b. is Spisula solida
. BTW the hole at the tip of the S. subtruncata
is where either a Whelk, or more likely a Necklace Shell has bored in to predate the mollusc
Ooops. I've just noticed you mention predation by Necklace Shell in the other post about Donax vittatus