Let me begin this post by stating that I am NOT in the habit of "collecting" artifacts from conservation areas, but knowing Fred's interest in molluscs, and wanting to find out more about these freshwater clams, made this exception. There was a cold wind blowing, and I didn't have my measuring tape with me, or I probably would have just photographed these clam shells onsite.
I was walking my dog at the Little Cataraqui Creek Conservation Area, at the south end of the dam/reservoir, and noticed some clam shells. I poked along the dam and found a few that seemed to be exceptionally large. I have found these clamshells in other locations, but none that I recall being quite this size. The smallest measures about 3 1/2 inches long, the largest just a hair under 6 inches, with a circumference of just under 8 inches.