PLACUNA. 227 



to procure food for both. On his return he knocks at the shell, 

 which is opened to receive him, and they share the supplies 

 together in convivial security ! Some species are smooth, although 

 the greater number are imbricated or crisped outside. P. saccata, 

 fig. 162. 



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 Mu- 

 rex 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. (7r\aKouQ,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, con- 

 taining 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, 



a 2 



