228 



PLANAXIS. 



called the Saddle Oyster, from the anterior margin being turned 

 up so as to resemble a saddle. The genus may be known from all 

 others by the diverging costaon the hinge. Placunanomia is the 

 only genus resembling it in this respect, but this is easily dis- 

 tinguished by a perforation through the shell. Fig. 184, P. 

 Placuna. These shells are used in China to glaze windows. 



PLACUNANOMIA. (Sw. Placuna and A nomia.) Fam. Ostracea, 

 Lam. and Bl.— Descr. Thin, foliaceous, compressed, sub-equi- 

 valve, 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, con- 

 taining the cartilage, in the other ; muscular impressions one in 

 each valve, central, sub-orbicular. — Ohs. The specimens from 

 which Mr. Broderip described this singular genus, were brought 

 by Mr. Cuming from the gulf of Dulce in Costa Rico. Another 

 species is from one of the Philippine Islands. They partake of 

 the characters of several genera, having the hinge of Placuna, and 

 being attached by a process passing through the lower valve, like 

 Anomia. P. Cumingii, fig. 189. 



PLAGIOSTOMA. Sow. Min. Con. (TrXaytoe, plagios, oblique ; 

 GTOfm, stoma, mouth.) Fam. Pectenides, Lam. Palliobranchiata, 

 Bl. — Descr. Sub-equivalve, inequilateral, oblique, auriculated 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. 

 Fig. 1/6, P. spinosum. 



PLAIT or FOLD. A term applied to the prominences on the colu- 

 mellarlipof some univalve shells, particularly in the sub-family of 

 Volutidee. Ex. Voluta, fig. 433; Cymba,434; and Melo, fig. 435. 



PLANARIA. Brown. A minute fossil resembling Planorbis in 

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

 reflected outer lip. P. nitens, fig. 312, from Lea's Contributions 

 to Geology. 



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



