PLANAXIS. 83 
Pisidium, however, are more inequilateral than the Cyclades, and 
the posterior, or ligamentary side of the latter is the longer, 
while that of the former is the shorter. Fig. 112. 
PITONEL'LUS. Montf. Roresuua, Auct. 
PLACEN’TA, Schum. Puacuna, Auct. 
PLACU'NA. Brug. (maxis, placus, a cake.) Fam. Ostracea, 
Lam. and Bl. Descr. Compressed, thin, equivalve, nearly equi- 
lateral, planorbicular, fibrous, foliaceous; hinge flat, with two 
diverging ribs in one valve, corresponding with two diverging 
grooves in the other, containing the cartilage; muscular impres- 
sions one, large, circular, central, and one or two smaller in each 
valve. Obs. The two best known species of this well defined 
genus are the P. Placenta, commonly called the Moon Shell, 
and the P. sella, called the Saddle Oyster, from the anterior mar- 
gin being turned up so as to resemble a saddle. The genus may 
be known from all others by the diverging costa on the hinge. 
Placunanomia is the only genus resembling it in this respect; 
but this is easily distinguished by a perforation through the 
shell. Fig. 184, P. Placenta. 
PLACEN’TULA. Schum. A genus of microscopic Foraminifera. 
PLACUNANOMIA. Sow. ( Placuna and Anomia.) Fam. Os- 
tracea, Lam. and Bl. Deser. 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 diverg- 
ing grooves, containing the cartilage, in the other; muscular 
impressions one in each valve, central, sub-orbicular. Obs. The 
specimens from which 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 Phillippine 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. 
PLAGIOS'TOMA. Sow. (mrayios, plagios, oblique; stopa, stoma, 
mouth.) Fam. Pectinides, Lam. Palliobranchiata, Bl. Deser. 
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. In Bath Lyas and Chalk, &c. Fig. 176, P. 
Spinosum. 
PLANARTA. Brown. A minute fossil genus resembling Pla- 
norbis in appearance, but differing in being a marine shell, and 
having a reflected outer lip. P. nitens, fig. 312, from Lea. 
PLANAX'S. Lam. (Plana, flat and axis.) Fam. Turbinacea, 
Lam. Entomostomata, Bl. Descr. Sub-ovate, pyramidal, solid ; 
spire measuring 3 or 4 of the axis, consisting of few whorls; 
