POLYGONUM. Schum. ([Greek: Polus], _polus_, many; [Greek: gônia], _gonia_, an angle.) A genus composed of species of TURBINELLA, Auct. which have large continuous costæ, so as to present the appearance of many-sided shells. T. polygonus, fig. 383. This generic name may be used to include all those species of Turbinella, Auct. which have very small folds on the columella. POLYGYRA. Say. A genus of Heliciform shells, characterized by the large number of close set whorls, constituting the spire. _Ex._ P. Septemvolvus, fig. 275, 276. POLYLEPAS. Bl. ([Greek: Polus], _polus_, many; [Greek: lepas], _lepas_, rock.) SCALPELLUM, Auct. POLYMORPHINA. D'Orb. A genus of microscopic Foraminifera. POLYPHEMUS. Montf. A genus composed of species of ACHATINA, Auct. which have elongated apertures, short spires, and an undulation in the outer lip. P. Glans, fig. 288. POLYPLAXIPHORA. Bl. The second class of the sub-type Malentozoa, Bl. containing the genus Chiton. POLYSTOMELLA. Lam. A genus of microscopic Foraminifera. POLYTHALAMACEA. Bl. ([Greek: Polus], _polus_, many; [Greek: THalamos], _thalamos_, chambers.) The third order of Cephalophora, Bl. the shells of which are described as straight, more or less symmetrically convolute, divided into several chambers. The septa are sometimes, but not always, pierced by one or more siphons. This order is divided into the families, Orthocerata, Lituacea, Cristacea, Ammonacea, Nautilacea, Turbinacea, Turriculacea, all of which contain genera of chambered shells. De Blainville arranges these families according to the degree in which the spires revolve. The first being straight, as the Orthocerata, and the last being so closely coiled up, that the last whorl covers the rest, as in the Nautilacea. POLYTHALAMIA. Lam. The first division of the order Cephalopoda, Lam. containing the following families of chambered shells, viz. Orthocerata, Lituacea, Cristacea, Sphærulacea, Radiolata, Nautilacea, Ammonacea. Fig. 463 to 484. POLYTROPA. Sw. A genus of "Scolyminæ," Sw. thus described: "Bucciniform; but the base narrow, and ending in a straight and contracted, but rather short, channel; spire longer, or as long as the aperture; exterior folliculated, or tuberculated; inner lip flattened, as in _Purpura_; basal notch small, oblique; no internal channel; crispata. En. Méth. 419, f. 2. Chem. 187, f. 1802. Capilla, Pennant, pl. 72, f. 89, imbricata. Mart. 122. f. 1124. ? rugosa. Chem. f. 1473-4." Sw. p. 305. POLYXENES. Montf. A genus of microscopic Foraminifera. POMATIA. Gesner. (Gray, Syn. B. M. p. 133.) A genus of the family of "Cyclostomidæ," described as having "an elongated shell with reflexed lips, and a horny spiral operculum." Also a sub-genus of Snails, containing HELIX pomatia, Auct. (Gray's Turton, p. 135.)