Need ID

[paul monfils]

The commonest rock-boring clam in your vicinity is Rupellaria typicum, which I believe is included in the Peterson guide.  If you can extract one of the clams from the hole without breaking it, compare it to the picture.  If it is that species, the ones you described would be young ones.  It grows to about three times that size.  You could also check out Petricola pholadiformis.  Most of the other boring clams would usually occur either pretty far north or pretty far south of you.  Are you sure the matrix is rock, and not a thick piece of a large shell?  Such material often gets blackened after being buried in muddy sediment for a while.