GASTEROPODA. Lam. ([Greek: Gastêr], _gaster_, belly; [Greek: pous, podos], _pus_, _podos_, a foot.) The second order of the class Mollusca, Lam. containing those molluscous animals whose organs of locomotion are ventral. Most of the shells belonging to this order are patelliform, placed upon the back of the animals, which rest or crawl upon the belly. This order is divided into Pneumonobranchiata, that is, those which breath air, or land molluscs; and Hydrobranchiata, or those which breath water, marine or fresh-water molluscs. Fig. 227 to 263. GASTRANEA. Schum.? CORBULA, Auct. GASTROCHÆNA. Speng. ([Greek: Gastêr], _gaster_, belly; [Greek: chaino], _chaino_, gape.) _Fam._ Pholadaria, Lam. Pyloridea, Bl.--_Descr._ Equivalve, regular, inequilateral, with a wide, oblique, ventral hiatus, enclosed in a curved pyriform tube. Differing from Galeomma in being a free, oblique shell; from Fistulana, in the oval shape of the valves, and the curve of the tube; from Aspergillum and Clavagella, in both valves being free.--_Obs._ The Gastrochænæ are found in the hollows of massive shells or other marine substances. Fig. 62, G. Modiolina. GASTROPLAX. Bl. UMBRELLA, Lam. De Blainville described this genus from a specimen in which the shell had been, probably by accident, placed upon the under part of the animal, and not discovering his error until afterwards, gave it the above name. GEOMITRA. Sw. A sub-genus of Geotrochus, Sw. founded on a trochiform species of Helix, with coronated nodules on the whorls. Helix bicarinata, Sow. Zool. Journ. 1, pl. 3, fig. 7. Sw. page 166 and 332. GEOPHONUS. Montf. Conch. Syst. t. 1, p. 19. A genus of microscopic Foraminifera. GEOTROCHUS. Sw. HELIX pileus, Auct. (fig. 278,) and other trochiform species. Divided into the sub-genera Pithohelix, Geotrochus, Hemitrochus, Gonidormus, and Geomitra. Sw. p. 165 and 166, described at page 331. GEOVULA. Sw. A sub-genus of Melampus (Auricula), consisting of oval species, resembling Auricula Midæ, fig. 297. GERVILLIA. Defr. _Fam._ Margaritacea, Bl. Malleacea, Lam.--_Descr._ Equivalve, oblong, oblique; hinge long, straight, having small, irregular, transverse ligamentary pits.--_Obs._ This genus of fossil shells, found at various geological periods, from the Lias to the Baculite limestone in Normandy, is now extinct. In general form it resembles Avicula, but in the hinge it approaches Perna. Fig. 169, G. Avicularis. GIBERULA. Sw. A genus separated from MARGINELLA, Auct. and thus described, "sub-oval; spire slightly prominent; top of the outer lip dilated and gibbous; base of the inner lip with plaits; inner lip broad, spreading. G. Zonata. Enc. Méth. 374, f. 6."