Mollusc taxonomy

Conchifera is a subphylum of the phylum Mollusca, containing five extant classes: Monoplacophora, Cephalopoda, Gastropoda, Bivalvia, and Scaphopoda. Conchiferans can bear a single shell as in snails and ammonites, a single pair of shells as in clams, or lack a shell as in slugs and squid. The other subphylum is Aculifera, the members of which are shellless or have a row of several plates. Non-monoplacophoran conchiferans emerged within the once-widespread Monoplacophora. The only descendant which retains its ancestral shape is the Tryblidiida.

Aculifera (older name: Amphineura) is a clade of molluscs incorporating those groups that have no conch or shell, that is, the Polyplacophora, Caudofoveata and Solenogastres. It is a sister group to the Conchifera. Monophyly of Aculifera is supported by fossil, anatomical and molecular studies.

Imparidentia is a major clade of bivalves named by Bieler, Mikkelsen, and Giribet in 2014; this clade, commonly recovered in phylogenetic analyses, consists of all euheterodonts other than the anomalodesmatans. The name comes from Latin impar "unequal" and dens "tooth", in reference to the unequal hinge teeth seen in members of the clade. The relationships among members of Imparidentia are uncertain.