Univalve - A univalve is a shell that contains one valve.
Uni = One
An organism that fits into the category of univalve would be a snail. Snails have one singular shell that is only composed of one valve.
Bivalve - A bivalve is a shell that contains two valves.
Bi = Two
An organism that fits into the bivalve category would be a scallop. Scallops have two valves instead of one.