PLACUNA. 



TESTA bivalvis^ libera^ suborbiculariSj subaequi- 

 valviSj complanata. Cardo interior^ cicatriculis 

 diiabuSj basi convergentibus, superne divaricatis 

 in valva inferiore ; costis duabus elongatis^ longi^ 

 tudine inaequalibus^ minus divaricatis in alteraj 

 ligamento praecipue inservientibus. Impressio 

 muscularis unica, simplex^ suborbiculq^ris, 



Easily distinguished from all other Genera by the two 

 elongated, diverging costce, or laminae, of unequal length, 

 to the external edges of which the mternal ligament is at- 

 tached, in the one valve; and by the two corresponding, 

 but more divaricate, impressions in the other valve. All the 

 species known tons at present,(which are not numerous), are 

 extremely compressed, suborbicular, and nearly equivalve, 

 the valve which has the two cardinal costae, commonly 

 called the upper valve, being rather less concave than the 

 other. The valves are very thin, more or less transparent, 

 of a foliated structure, composed of perpendicular fibres 

 as in Ostrea and Pinna, A single, nearly circular, muscu- 

 lar impression is observable in the centre of each valve, 

 with sometimes a minute lunulate, more lateral one and 

 less frequently a third, very small, also lateral one placed 

 at nearly the same distance on the other side of the princi- 

 pal muscular impression. A part of the internal ligament^ 

 which unites the two valves at the hinge, spreads on both 

 sides the mnbo in a transversely linear form, acutely dentated 

 at its inner edge ; this, on account of the imperfect state in 

 which the specimens of this shell are generally brought to 

 Europe, is very seldom observable. Slender, very obsolete 

 striag, diverge from the umbo and cover the outside of both 

 valves. 



The recent species are inhabitants of the Indian seas : 

 three are enumerated by Lfamarck, of which the Anemia 



