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.