What you are seeing is just the next rows of scutes.
The mantle just isn't extended past the shell in this photo - could possibly be pulled in because of the photographer actually - they pull in quickly if they notice shadows and so on.
But I understand what you're saying. There are some really amazing patterns on clams out there. There very likely are clams with this sort of pattern on their mantles.
Some eye candy for you..
http://reefcentral.com/forums/showth...readid=1290440
http://canreef.com/vbulletin/showthread.php?t=41367&
Enjoy