Hi, Byerlyjb - and welcome aboard!
The snail might be one of the Cerith group - some of them have that sort of look . . .
The other almost looks like a limpet to me - if that limpet were covered with cyanobacteria, or red coralline algae. They've got that sort of fringe under the rim of their shells, and are basically very snail-like in most respects. Most of them will scrape algae from your rock, and to do that, they've got teeth made from one of the hardest substances known to science!
~Bruce, not a hitchhiker pro . . .