OLIVIDÆ ^Oliva^ ^O. literata^ ^O. mutica^ ^Olivella^ ^O. biplicata^ ^O. boetica^ CONIDÆ ^Conus^ ^C. floridanus^ ^C. pealii^ CANCELLARIIDÆ ^Cancellaria^ ^C. reticulata^ Class =SCAPHOPODA= DENTALIDÆ ^Dentalium^ ^D. dentale^ ^D. pretiosum^ ^D. hexagonum^ ^Entalis^ ^E. striolata^ [pg328] CLASS =GASTEROPODA= The class /Gasteropoda/ is the largest and most comprehensive subdivision of the /Mollusca/, and within its wide range of families many differing details of organization are to be found. As a class it possesses all the most characteristic features of the phylum, though it is frequently modified in a high degree. This class includes all the univalve mollusks (except /Nautilus/ and /Dentalia/), such as the snails, the whelks, and the host of spirally coiled land, fresh-water, and marine shells. The gasteropod foot is, as has been remarked, primarily the same as in our schematic mollusk—a flat, muscular disk caused by the thickening of the ventral body-surface. The neck, head, and tentacles are also quite the same, but considerable modification of these organs will be found when we come to examine some specimens. The mantle is always present, except in the nudibranch or non-shell-bearing forms; but in few of the /Gasteropoda/ is the mantle so regularly simple as in our ideal mollusk. Perhaps the most striking feature of the /Gasteropoda/, and one that will at first surprise him who has in mind the simple structure of the ideal mollusk, is the fact that they are always asymmetrical—that is to say, a median line drawn longitudinally through a gasteropod will not divide it into halves of similar anatomical structure.