while attending class my professor asked us to count benthic species,and in my counting i mentioned sea snails,certain i have read multiple times that their classification is "univalvia" since their shell is made from a single piece,unlike the two shells of BIvalvia. Now howerver,i see the term is used as arhaic?
Those mollusks that have shells, have only a single shell. Bivalves do not have "two shells", they have two valves or...BIvalvia. Thus gastropods were considered UNIvalvia (univalves). The clams, mussels, scallops are all bivalves or Bivalvia. The shelled shails are univalves but you are correct that snails are not taxonomically "univalvia" but are Gastropoda.
Univalvia also was used for shell-bearing cephalopod mollusks, not only for gastropods.
Thank you. I encountered the term when dealing with gastropods. So if i understood you correctly,the term was also used for Nautilidae?