That clam looks like a very interesting and unique squamosa to me. Except for the color, and scutes, the pattern is identical to my squamosa. The shell doesn't seem to be creased enough for a maxima. Maybe its a derasa; if it is, it's especially colorful and patterned. Most derasas I've seen aren't nearly as 'marbled'---unlike a teardrop clam, which looks nothing like this clam. Also, I've never seen gigas clams with their mantles much beyond the shell, either; even in Knop's book, there are very few pictures of gigas clams with their mantles past the edge of the shells.