88 CONCHOLOGY. 



4. Placuna. The Chinese Window Shell. Three species. 



This genus received its common name from the thin, 

 transparent nature of the valves of the shell, particularly of 

 the species placenta, which by the ingenious Chinese are 

 often polished and used as a substitute for window-glass. 



The hinge of the shells of this genus is so peculiar as to 

 make it perfectly distinct ; entirely interior, fastened by a 

 ligament shaped like a V on one of the valves. 



Shell free, sub-irregular, very thin, almost entirely trans- 

 parent, flat, sub-equivalve, sub-equilateral, slightly auricled ; 

 hinge entirely internal, formed on the superior less valve 

 by two elongated, unequal, oblique ribs converging at the 

 summit, to the interior side of which a ligament like the let- 

 ter V is inserted in two equally converging, rather deep 

 cavities of the lower valve, which is more convex ; one 

 rather small, sub-central muscular impression. 



P. 'placenta. The glassy Placuna. PI. 16, fig. 3. 



Sub-orbicular, flat, white, and transparent ; finely striated 

 longitudinally, slightly decussated. 



5. Anomia. The Antique Lamp. Six species. 



When Linnffius formed this genus and named it Anomia, 

 he probably did so from its having no determinate charac- 

 ter. Its common name was given it by the fancied resem- 

 blance of some of its species to an antique lamp. Like the 

 oyster, they seldom leave their place ; they are always affixed 

 to marine bodies by an osseous operculum, formed by the 

 thick extremity of the animal's muscle. The lower valve is 

 perforated and smaller, conforming to the shape of the sub- 

 stance to which it is affixed. 



Shell adhering, irregular, inequivalve, inequilateral, ostra- 

 ceous ; inferior valve rather more flat than the superior, di- 

 vided at the summit into two sloping branches, whose ap- 

 proaching together forms a large oval hole, through which 

 protrudes a muscle, the extremity of which becomes ossified 



