Until recently, I never knew what they were. Then on a Celery Farm walk with The Nature Conservancy, Barbara Brummer decided to find out -- by asking an expert (what a concept).
So, long story short, these mussels are an Alewife Floater (top) and an Eastern Floater (bottom).
Too bad no one else could solve our little mystery. (Thanks, Barbara!)
More on these native New Jersey fresh-water invertabrae here.