PINNATED. (From _Pinna_, a fin.) When a part of a shell is spread out and smooth, as in Rostellaria columbaria, fig. 403, it is said to be _alated_, or winged, but when the part which is spread is radiated or ribbed, like the fin of a fish, it is _pinnated_, as in Murex pinnatus, and Murex tripterus. (Conch. Illustr.) PIRENA. Lam. A genus of fresh-water shells, rejected by De Ferussac and other authors, who place Lamarck's two first species with Melanopsis, and his two last with Melania. Fig. 316, P. terebralis. PISIDIUM. Leach. A genus of river shells separated from Cyclas principally on account of a difference in the animal. The species of Pisidium, however, are less equilateral than the Cyclades, and the posterior or ligamentary side of the latter is the longer, while that of the former is the shorter. Fig. 112. PISIFORM. (_Pisum_, a pea; _forma_, shape.) Shaped like a pea or small globular body. PISUM. Megerle. (_A pea._) PISIDIUM, Leach. PITHOHELIX. Sw. A sub-genus of "Geotrochus," Sw. Sw. p. 332. PITONELLUS. Montf. ROTELLA, Auct. PLACENTA. Schum. PLACUNA, Auct. PLACENTULA. Schum. A genus of microscopic Foraminifera. PLACUNA. Brug. ([Greek: plakous], _placos_, a cake.) _Fam._ Ostracea, Lam. and Bl.--_Descr._ Compressed, thin, equivalve, nearly equilateral, planorbicular, fibrous, foliaceous; hinge flat, with two diverging ribs in one valve, and two corresponding grooves in the other, containing the cartilage; muscular impressions one, large, circular, central, and one or two smaller in each valve.--_Obs._ The two best known species of this well defined genus are the P. Placenta, commonly called the Chinese Window Shell, and the P. Sella, called the Saddle Oyster, from the anterior margin being turned up so as to resemble a saddle. The genus may be known from all others by the diverging costa on the hinge. Placunanomia is the only genus resembling it in this respect, but this is easily distinguished by a perforation through the shell. Fig. 184, P. Placuna. These shells are used in China to glaze windows.