you are correct it is the mantle thats the colorful part. the byssal is on the bottom, holds the clam to the substrate when it is young. Yeah see if you can shrink your pic down and atleast put it in your gallery so we could see it. Also I believe the snails bore through the actual clam shell, not the mantle but I could be wrong. Or do you have a "window" in the color? which means a loss of the symbiotic algae making it clear? Those usually appear to be round also.
chris