PEESICULA. 237 



■PEEIPLOMA. Schum. Fam. Myarise. A genus thus described : 

 " Shell very thin with the left valfe more ventricose than the 

 right; hinge toothless, ligament double, the external portion 

 thin, the internal part thick, placed upon prominent, sometimes 

 spoon-shaped hinge laminae, and supported by a transverse bone ; 

 muscular impressions two, distant, palleal impression sinuated 

 posteriorly." Genus, Osteodesma, Deshayes. Ex. P. insequi- 

 valvis. PI. in. fig. 72. 



PEEISTOMATA. Lam. A family belonging to the first section 

 of the order Trachelipoda, containing the following genera : — 



1. AMPTJLiiAETA. Griobose or discoidal ; operculum concentric ; 



including Pachystoma, Latiistes, Ceratodes. Pig. 318 

 to 320. 



2. Palitdina. Oval; operculum concentric. Pig, 321. 



3. Yaltata. Griobose ; operciilum spiral. Pig. 322, 

 PEEISTOME, The edge of the aperture, including the inner and 



outer lips, 



PEEITEEME, A term used to express the whole circumference 

 of the aperture of a spiral shell. In descriptions, it is said to be 

 notched or entire, simple, reflected, round or oval, &c. as the 

 case may be. 



PEELAMATEE. Schum. (Mother of Pearl.) Meleageiita 

 Margaritifera, Lam. The pearl oyster. 



PEENA. Auct. (" Pernae concharum generis," Phn.) Fam. 

 Malleacea, Lam. — Bescr. Sub-equivalve, irregular, compressed, 

 foHaceous ; hinge straight, linear, composed of a series of trans- 

 verse, parallel grooves, containing the cartilage and intermediate 

 spaces bearing the ligament ; anterior margin with a sinus for 

 the passage of a byssus ; posterior ventral margin oblique, 

 attenuated. Obs. This genus is known from Crenatula by the 

 straightness, number and regularity of the grooves in the hinge 

 and the sinus, for the passage of the byssus, P. Ephippium. 

 PL ix. fig. 166. Thesaurus Conchyliorum, PL 97, fig. 189 to 191. 



PEESICULA. Schum. A genus formed of Maeginella Persicula, 

 Auct. and other species having the spire concealed. PL xx, 

 fig. 438. 



