CLASSIFICATION. Gastropods are divided into two subclasses: (1) Streptoneura, "in which the visceral commissures are twisted into a figure of 8, and in which the sexes are distinct;" and (2) Euthyneura, in which the visceral com missures are not so twisted, and in which the sexes are united. The former contains the order Aspidobranchia, which includes the limpets, ear shells, top-shells, turban-shells, etc.; and the order Pectinibranchia, which contains the bulk of the other marine shell-bearing forms. The latter subclass also has two orders: Opistho branchia, containing the sea-hares (Aplysia), pelagic pteropods, etc.; and Pulmonata, contain ing the land and fresh-water air-breathing snails and slugs.
Fossil, GASTROPODS. Gastropod shells are found in all the geological formations from those of lowest Cambrian age to those of recent time, and they occur usually in abundance in those formations above the Cambrian. The earliest forms are limpet-like shells (Scenella) and a capulid (Stenotheca) in the Olenellus zone of the Lower Cambrian system. Very soon, in the Upper Cambrian a few turreted gastropods ap pear (Raphistoma and Straparollina). These true gastropods are in the Cambrian associated with a host of slender conical shells, the hyoli aids, often classed with the pteropods, but which should more properly be placed with the tubic olar worms.
In the Ordovician the gastropods are widely differentiated and are represented by numerous genera and abundant individuals, with such well known forms as Pleurotomaria, Bellerophon, Raphistoma, Murchisonia, Maclurea, Euomplia lus, and others. In the Silurian a further evo lution has taken place, manifested principally in the increased ornamentation of genera that come up from the Ordovician, and in the crea tion of new genera from those already existing. Some of the important forms are Loxonema, Murchisonia, Platyostoma, Pleurotomaria, Bu cania, Trematonottts, Etiomphalus. The Devonian formations are still richer in species and are char acterized by such forms as Loxonema, Turbo, Enomphalus, Platyostoma, Platyceras, .Acroculia, Macrocheilus. In the Carboniferous the same genera are present with the addition of Naticop sis, Vermetus, and Actreonina. The Permian gas tropod fauna is about the same as that of the Carboniferous. The majority of the Paleozoic gastropods belong to the more primitive, less specialized subclass of the Streptoneura, and especially to the order Aspidobranchia, and it is worthy of note also that the Paleozoic genera are as a rule holostomatous, i.e. they have shells with
non-siphonate apertures.
The Triassic formations at the beginning of the Mesozoic show important changes in the gas tropod fauna. The Paleozoic pteropods have dropped out; the Belleropliontidir. the Devonian Platyceridre and Platyostomithe have disappeared, and the euomphalids have become less abundant, and a new type of shell, the siphonostomatous, appears with the families Cerithiidm and Melanii dm, in which the siphon is, however, shorter than in the later members of these families. The important genera are Chemnitzia, Loxonema, Rissoa, Eulima, Trochus, Turbo, Pleurotomaria, Cerithium, Helcion. In the Jurassic the Valva tidm, Viviparidn, Melaniidn, Aporrhaidm, Strom bidle, Columbellidm, Cyprnidn begin their exist ence and the fauna is strongly siphonostomatous. One Jurassic family, the Nerineidu, which began in the Trias and continued into the Cretaceous, is a very characteristic Mesozoic shell, that may be recognized by its slender turreted spire, re sembling that of its allies the Cerithiidn, and by the peculiar longitudinal septa that project from the columella and walls of the whorls into the central cavity of the shell.
The Cretaceous ushers in another lot of fami lies: Solariidx, Cassididu, DoRiche, Tritonidu, Buccinidm, MuricidEe, Purpuricke, Volutidm, CancellaridEe, Pleurotomidu, Conidn, in fact, all the more specialized families of the Ctenobranchiata, including more pronounced siphonate forms. The gastropods hold third rank among the Cretaceous mollusks, being excelled by the clams and cephalopods. In the Tertiary the gastropods rise to first rank. Among the few new families appearing in the Tertiary the more important are the Harpidm and Ovulithe. The siphonostomate shells attain here their highest development, and are more prominent than any others. All the Tertiary forms are closely allied to modern forms; indeed, the majority of the Pliocene fossils and a small per cent. of the Miocene species are still living in the modern ocean. At the present day the gastropods are enjoying rapid progressive evolution.