222 PETRICOLA. 



PERIPLOMA. Schum. Fam. Myarise. A genus thus described : 

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

 right ; hinge toothless, ligament double, the external portion thin, 

 the internal part thick, placed upon prominent, sometimes spoon- 

 shaped hinge laminfe, and supported by a transverse bone ; mus- 

 cular impressions two, distant, palleal impression sinuated pos- 

 teriorly." Ex. P. insequivalvis. fig. 72. Genus, Osteodesma, 

 Deshayes. 



PEFJSTOMATA. Lam. A family belonging to the first section of 

 the order Trachelipoda, containing the following genera : — 



1. Amptjllaria. Globose or discoidal ; operculum concentric ; 



including Pachystoma,Lanistes, Ceraiodes. Fig. 318 to 320. 



2. Paludina. Oval ; operculum concentric. Fig. 321. 



3. Valvata. Globose ; operculum spiral. Fig. 322. 

 PERISTOME. The edge of the aperture, including the inner and 



outer lips. 



PERJTREME. A term used to express the whole circumference 

 of the aperture of a spiral shell. It is said to be notched or 

 entire, simple, reflected, round or oval, &c. 



PERLAMATER. Schum. {Mother of Pearl.) Meleagrina 

 Margaritifera, Lam. The pearl oyster. 



PERNA. Auct. (" Pernae concharum generis," Plin.) Fam. 

 Malleacea, Lam. — Descr. Sub-equivalve^ irregular, compressed, 

 foliaceous ; 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. Fig. 166, P. 

 Ephippium. Mostly tropical. 



PERSICULA. Schum. A genus formed of Marginella Persicula, 

 Auct. and other species having the spire concealed. Fig. 438. 



PERSONA. Montf. {Mask). A genus composed of Triton 

 Anus, Auct. and similar species. Fig. 401. 



PETRICOLA. Lam. {Petrus, a stone ; cola, an inhabitant.) Fam. 



