296 OSTREIDA. 
entirely of subnacreous, plicated laminz, peculiarly separable, 
and occasionally penetrated by minute tubuliimCARPENTER. 
P. sella, called from its shape the “ saddle-oyster,” is remark- 
ably striated. 
Placuna is essentially like Anomia, having the generative 
system attached to the right mantle-lobe. and the ventricle 
exposed. The mantle-margin is cirrated, and furnished with a 
curtain, as in Pecten; the foot is tubular and extensile, the 
small muscular impressions before and in the rear of the adductor 
are produced by suspensors of the gills. 
PLACENTA, Auct. (Not Retzius, 1788 = Placuna. Placunema, 
Stoliezka, 1870.) Shell thin, suborbicular, semitransparent ; 
cartilage-erooves and lamelle slightly divergent, the posterior 
longest; muscular impression subcentral. P. sella, Gmel. 
(Exxxi78). China. 
PSEUDOPLACUNA, Mayer, 1876. Shell lenticular, rather thick, 
almost smooth and nearly equivalve; upper valve swollen ; mus- 
cular impression large, round, central, approaching the hinge ; 
hinge-lamelle strongly diverging, dissimilar. P. Helvetica, 
Mayer. Eocene; Eur. 
SAINTIA. Raineourt, 1877. Shell small, rounded, smooth; 
muscular impression large, approaching the posterior margin ; 
hinge with two diverging lamelle enclosing a third very small 
tooth. S. Munieri, Raincourt (exxxii, 10,11). Fossil; Paris 
Basin. 
HeEmIPLICATULA, Desh., 1864. 
Distr.—H. solida, Desh. (cxxxiii, 32, 33). Fossil; Paris 
Basin. 
Shell roundly oval, solid, compressed, subequivalve, hinge 
with two slightly diverging hinge-ribs in each valve, those of 
the right valve fitting between those of the left, which are less 
elevated and have between them a small fosset; the cartilage is 
attached, as in Placuna, along the external sides of the hinge- 
ribs, and this forms the principal distinction between the present 
genus and Plicatula, where ‘the cartilage is situated in the 
median pit. 
? piconiuM, Meyer, 1880. B. irregulare, Meyer. Oligocene; 
Germany. 
( Ostracea.) 
Famity OSTREID%. 
Shell inequivalve, slightly inequilateral, free or adherent, 
resting on one valve; beaks central, straight; ligament in- 
ternal; epidermis thin; adductor impression single, behind the 
centre; pallial line obscure; hinge usually edentulous. 
Animal marine; mantle quite open; very slightly adherent 
