the pattern kinda suggests derasa, but i'm seeing scutes and ribs on the shell, which suggest otherwise. also, there are no irridipores (light sensing organs) around the edge of the mantle (check Joe's pic of his derasa). i'm inclined to say squamosa at this point, maybe maxima.
can you shoot a pic of the clam from the side, showing the shape of its shell and a close-up of the incurrent siphon?