In all animals, as a general rule, those organs by which they obtain their food belong to the first class; hence the characters of the suborder and its divisions have been taken from the form of the mouth and the disposition and form of the teeth on the lingual membrane, as I have no doubt these prats have a most important bearing on the oeconomy of the animal; while the characters of the families have been taken from the modifications of the mantle and differences in the structure and form of the operculum and shell; for though I consider that the characters of the order, suborder, and families should be chiefly taken from modifications of the animal, I always consider that the shell and operculum are quite as important as regards the genus, as the animal which forms it."--introduction.