Xenoconchia

a group of extinct mollusks, close to the gastropods and monoplacophors. They have been found in Carboniferous and Permian deposits. Xenoconchia have a high conical shell, open only at the broad end and lacking any sort of internal dividing structure. The height of the shell reaches 10 cm. The group includes one order, Toxeumophorida, which unites two genera (with 4–5 species). It is of interest for explaining the evolution of ancient mollusks.