296 OSTREID^. 



entirely of subnacreous, plicated latninre, peculiarly separable, 

 and occasionally penetrated by minute tubuli. — Carpenter. 



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 ciirated, 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 bj^ suspensors of the gills. 



PLACENTA, Auct. ( Not Rctzius, 1788 = Placuna. Placunema, 

 Stoliczka, 1870.) Shell thin, suborbicular, semitransparent ; 

 cartilage-grooves and lamellse slightly divergent, the posterior 

 longest ; muscular impression subcentral. F. sella, Gmel, 

 (cxxxi, 78). China. 



PSEUDOPLACUNA, Maj'cr, 1876. Shell lenticular, rather thick, 

 almost smooth and nearly equivalve ; upper valve swollen ; mus- 

 cular impression large, round, central, approaching the hinge; 

 hinge-lamellai strongly diverging, dissimilar. P. Helvetica, 

 Mayer. Eocene ; Eur. 



SAINTIA. Raincourt, 1877. Shell small, rounded, smooth; 

 muscular impression large, approaching the posterior margin ; 

 hinge with two diverging lamellae enclosing a third very small 

 tooth. S. Munieri, Raincourt (cxxxii, 10, 11). Fossil; Paris 

 Basin. 



Hemiplicatula, 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. 



i* BICORIUM, Meyer, 1880. B. irregulare, Meyer. Oligocene ; 

 Germany. 



( Ostracea.) 

 Family 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 



