piAcuNA. 243 



PINNATED. (From Pinna, a fin.) Wlien a. protuberant part of a 

 shell is spread out and flattish, as in Eostellaria columbaria, fig. 

 403, it is said to be alated, or winged, but when tbe protuberant 

 part is radiated or ribbed, like the fin of a fisb, it is pinnated, 

 as in Murex pianatus, and Murex tripterus. (Conch. Illustr.) 



PIEENA. Lam. A genus of fresh-water shells, rejected by De 

 Perussac and other authors, who place Lamarck's two first 

 species with Melanopsis, and his two last with Melania. P. te- 

 rebralis, PL xiv, fig. 316. 



PISIDIUM. Leach. A genus of river sheEs 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. PI. v. 

 fig. 112. 



PISIFOEM. (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 " aeotrochus," Sw. Sw. 

 p. 332. 



PITONELLUS. Montf. Eotella, Auct. 



PLACENTA. Schum. Placuna, Auct. 



PLACENTUL A. Schum. A genus of microscopic Poraminifera. 



PLACUNA. Brug. (jrXaKovg, plaeos, a cake.) Fam. Ostracea, 

 Lam. and Bl. — Bescr. Compressed, thin, equivalve, nearly equi- 

 lateral, 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 



K 2 



