When you say it was chiton-like, do you mean the upper side, or the underside? If the upper side (the "shell" in other words) appeared "segmented", then it most likely was not a mollusk. More likely an arthropod, either an insect or a crustacean - perhaps an amphipod or something similar. However, if the shell was not segmented, and the underside had a foot like a chiton, then it is probably one of the fresh water limpets, family Acroloxidae. Paul M.