Jim Movich asked for some help identifying scallops attached to seaweed.
 
Kelp pectens (Leptopecten spp.) live attached to large seaweeds off
California, and they have the characteristics that Jim described. They also
have quite a nice fossil record; my first term paper in paleontology
(1973?) was on their fossil relatives. The fossils sometimes occur in thin
shell beds consisting of nothing but these scallops.
 
Andrew K. Rindsberg
Geological Survey of Alabama