PLACUNANOMIA. (Sw. _Placuna_ and _Anomia_.) _Fam._ Ostracea, Lam. and Bl.--_Descr._ Thin, foliaceous, compressed, sub-equivalve, sub-equilateral, irregular, flat near the umbones, plicated towards the margins, attached by a bony substance passing through a fissure in the lower valve; hinge flat, with two diverging ribs in one valve, corresponding with two diverging grooves, containing the cartilage, in the other; muscular impressions one in each valve, central, sub-orbicular.--_Obs._ The specimens from which Mr. Broderip described this singular genus, were brought by Mr. Cuming from the gulf of Dulce in Costa Rico. Another species is from one of the Philippine Islands. They partake of the characters of several genera, having the hinge of Placuna, and being attached by a process passing through the lower valve, like Anomia. P. Cumingii, fig. 189. PLAGIOSTOMA. Sow. Min. Con. ([Greek: plagios], _plagios_, oblique; [Greek: stoma], _stoma_, mouth.) _Fam._ Pectenides, Lam. Palliobranchiata, Bl.--_Descr._ Sub-equivalve, inequilateral, oblique, auriculated on each side of the umbones, radiately striated; hinge straight in one valve, with a triangular notch in the other.--_Obs._ This genus, one species of which is spinous, and another smooth, is only known in a fossil state. It is found in the Lias and chalk. Fig. 176, P. spinosum. PLAIT or FOLD. A term applied to the prominences on the columellar lip of some univalve shells, particularly in the sub-family of Volutidæ. _Ex._ Voluta, fig. 433; Cymba, 434; and Melo, fig. 435. PLANARIA. Brown. A minute fossil resembling Planorbis in appearance, but differing in being a marine shell, and having a reflected outer lip. P. nitens, fig. 312, from Lea's Contributions to Geology. PLANAXIS. Lam. (_Plana_, flat; and _axis_.) _Fam._ Turbinacea, Lam. Entomostomata, Bl.--_Descr._ Sub-ovate, pyramidal, solid; spire measuring ½ or 1/3 of the axis, consisting of few whorls; columella contiguous to the axis, flat, truncated, and separated from the outer lip by a short canal; outer lip thickened and denticulated within; operculum horny, thin, with a terminal nucleus.--_Obs._ This is a genus of small marine shells found in the West Indies, &c. Fig. 365, P. sulcata. PLANE. (_Planus._) Flat, planed, as the columellar lip of Purpura, fig. 414. PLANORBICULAR, (_Planus_, flat; _orbis_, an orb.) Flat and circular, as Ammonites, fig. 478.