PERIOSTRACUM. 221 



PENTAMERUS. Sow. (news, pente, five ; iiepog, meros, part.) 

 Fam. Brachiopoda, Lam. — Descr. Equilateral, inequivalve ; 

 one valve divided by a central septum into two parts ; the other 

 by two septa, into three parts ; umbones incurved, imperforate. — 

 Obs. Dalman remarks upon his genus Gypidia, that it is most 

 probably identical with Pentamerus, Sow. but rejects the name 

 for two reasons ; 1st. That it has already been applied to a 

 class of insects ; 2nd. He disputes the fact of the shell being 

 quinquelocular, i. e. not counting the triangular foramen in the 

 hinge of the larger valve as one of the divisions. Fig. 212, 213. 



PENTELASMIS. Leach. {Uevte, pente, five; g'Xao-jua, elasma, plate.) 

 Order. Pedunculated Cirripedes, Lam. — Descr. Compressed, 

 conical, composed of five valves ; lower lateral pair sub-trigonal ; 

 upper lateral pair elongated, sub-quadrate ; dorsal valve arcuate, 

 peduncle elongated, smooth. Found on floating wood in the sea. 

 — Obs. This genus is known from all others of the order by the 

 number of valves. Pentelasmis is the genus Anatifera of La- 

 marck. Lepas anatifer, Linn. Fossil species of this marine 

 genus are found in the Calcaire-grossier of Paris, and in other 

 similar beds. Fig. 34, P. laevis. 



PENULTIMATE WHORL. The last whorl but one. 



PERA. Leach. A genus composed of Cyclas amnica, and other 



similar species. 

 PERDIX. Montf. Dolium Perdix, Auct. 



PERFORATED. ( Perforates.) Bored through, as the apex of 

 Fissurella, fig. 245, and Dentalium, fig. 2. 



PERFORATION. {Perforo, to bore, or pierce,) A round opening, 

 having the appearance of being bored, as in Haliotis, fig. 338. 

 Sometimes the term is applied to an umbilicus which penetrates 

 a shell through the axis to the apex, as Eulima splendidula, fig. 348. 



PERIBOLUS. Brug. A genus founded upon young specimens of 

 Cypr^ea, with their outer lips not formed. 



PERIOSTRACUM. A name used by Mr. Gray to signify the sub- 

 stance which covers the outer surface of many shells, called 

 the Epidermis by most conchological writers. " Drap Marin" is 

 the name given to this substance by French Naturalists. 



