244 PL AK AXIS. 



the shell. These -shells are used in China to glaze windows, 



■ PL xi. 184, P. Placuna. 



PLACUNANOMIA. (Sw. Placuna and Anomia.) Fam. Os- 

 tracea, Lam, and Bl. — Bescr. Thin, foliaceous, compressed, sub- 

 equivalve, sub -equilateral, irregular, flat near the umbones, 

 plicated towards the margins, attached by a bony substance 

 passing through a fissure in the lower valve ; hinge flat, with 

 two diverging ribs in one valve, corresponding with two diverging 

 grooves, containing the cartilage, in the other ; muscular impres- 

 sions one in each valve, central, sub-orbicular. — Obs. The 

 specimens from which Mr. Broderip described this singular 

 genus, were brought by Mr. Cuming from the gulf of Dulce in 

 Costa E/ico. Another species is from one of the Philippine 

 Islands. They partake of the characters of several genera, 

 having the hinge like Placuna, and being attached by a process 

 passing through the lower valve, like Anomia. P. Cumiiigii, 

 PI. xi. fig. 189, 190, 191. 



PLAGIOSTOMA. Sow. Min. Con. {-irXayioe, plagios, oblique j 



. cTTOfia, stoma, mouth.) Fam. Pectenides, Lam. Palliobranchi- 

 ata, Bl. — Bescr. Sub-equivalve, inequilateral, oblique, auricu- 

 lated on each side of the umbones, radiately striated ; hinge 

 straight in one valve, with a triangular notch in the other. — Obs. 

 This genus, one species of which is spinous, and another smooth, 

 is only known in a fossil state. It is found in the Lias and 

 chalk. P. spinosum. PI. x. fig. 176. 



PLAIT or POLD, A term applied to the prominences on the 

 columellar lip of some univalve shells, particularly in the sub- 

 family of Volutidse. Ex. Yoluta, fig. 433 ; Cymba, 434 ; and 

 Melo, fig. 435. 



PLAJSTAEIA. Brown. A minute fossil resembling Planorbis in 

 appearance, but differing in being a marine shell, and having a 

 reflected outer lip. Prom Lea's Contributions to Geology. 

 P. nitens. PI. xiv. fig. 312. 



PLANAXIS. Lam. {Plana, flat ; and axis.') Fam. Turbinacea, 

 Lam. Entomostomata, Bl. — Bescr. Sub-ovate, pyramidal, solid ; 

 spire measuring f or i of the axis, consisting of a few whorls ; 



