Lamarck's Genera of Shells. 45 



Shell oblong, straight ; base attenuated ; internally purplish 

 white ; transverse concentric wrinkles ; longitudinally obsolete. 

 Indian Ocean. 7 Species. PI. II. Fig. 95. 



4. Placuna *. 



Shell free, irregular, flattened, subequivalve. Hinge internal, 

 with, on the upper valve, two sharp longitudinal ribs, close at the 

 base, and diverging in the form of the letter V ; on the other, 

 two ligamental impressions, corresponding to the cardinal ribs. 



The two oblong, prominent, rib-like laminae, in the form of a 

 V, situated at the internal hinge, on the upper valve of the shell, 

 is the essential character of this genus ; they serve for the at- 

 tachment of the ligament, inserted in the two impressions of the 

 same form observable in the opposite valve. The valves of the 

 placunse are thin, transparent, and of the same size. These 

 shells are large, orbicular, or subtriangular, sometimes trian- 

 gular, with only one internal muscular impression, like the 

 ostrese. Their texture is foliated. 



Type. Placuna sella t. (Anomia sella. Linn.) 

 Shell subquadrangular, curved, broad, irregularly sinuous, 

 lamellar, wavy ; bronze-coloured ; stria longitudinal, very fine. 

 Indian Ocean. 4 Species. PI. II. Fig. 96. 



5. Anomia J. 



Shell inequivalve, irregular, operculated, adhering by the 

 operculum; smaller valve perforated, usually flat, having a hole 

 or notch at the beak ; the other valve rather larger, concave 

 entire. Operculum small, elliptical, osseous, connected with 

 the internal muscle of the animal, and fixed to marine bodies. 



The operculum of the anomia has been absurdly mistaken for 

 a third valve, being, in reality, only the dilated and thickened 

 extremity of the tendon of the interior muscle of the animal, 

 which forms a small, solid, elliptical, and almost bony mass, of 

 such a shape as to fill the hole or notch of the beak of the 



• From ■B-Xa*, a broad table ? t A saddle, 



t AvofAM, from a. not, and vo^wo? law,— non-conformity to the usual order 

 anomalous. 



