Webster says:

bi•valve (1) "bi-'valv\ adjective: having a shell composed of two valves

bivalve (2) noun: an animal (as a clam) with a 2-valved shell

valve noun: 4: one of the distinct and usu. movably articulated pieces of which the shell of some shell-bearing animals consists

"Clamshell" is a metaphor. By definition 1, "bivalve" actually describes the lens construction. Technically, the lens is a bivalve lens, but it is known in the vernacular as a clamshell lens.

By definition 2, "bivalve" can also be used metaphorically, but it is more politically correct than "clamshell", since the latter excludes the shells of other bivalve mollusks such as mussels, scallops and oysters.

clam noun 1 a : any of numerous edible marine bivalve mollusks living in sand or mud

I hope this clears things up, but if it doesn't, blame it on the environment.

-Art [Happy as an edible marine bivalve mollusk living in sand or mud]
[This message has been edited by Art (edited 09-05-99).]