78 CONCHOLOGY. 



Shell. Subnacred, irregular, flat, elongated, subequivalve, 

 inequilateral, with summits anterior, distant and flexed inferiorly ; 

 hinge oral and toothless ; ligament undivided, thick, inserted in a 

 round pit excavated in a projecting apophysis upon each valve ; 

 a moderately large subcentral muscular impression, and tv^o very 

 small ones altogether anterior. Inhabits the Indian and Austral- 

 asian seas. Six living species. One fossil. 



Vulsella hians. Vulsella hngulata. 



V. rugosa. V. mytilina. 



V. spongiarura. V. sevata. 



4. Genus Placiina. 

 Jlnimal. Entirely unknown. 



Shell. Free, subirregular, very fine, almost entirely translucid, 

 flat, subequivalve, subequilateral, slightly auriculated ; hinge alto- 

 gether internal, formed upon the superior valve, which is the 

 smaller, by two elongated, unequal, oblique, crests ; converging to 

 the summit, at the internal side of which a ligament is attached in 

 the form of a V., a single, small, subcentral muscular impression. 

 Inhabits the Indian seas. Two fossils in France. Three living 

 species. 



Placuna sella. Placuna papyracea. 



P. placenta. 



5. Genus Anomia. 



Animal. Much compressed ; edges of the mantle very fine ; 

 not adhering, and furnished exteriorly with a row of tentacular 

 filaments; contractile muscle thick, divided into three parts, the 

 largest of which passes partially across a slope of the inferior valve 

 and often contains a calcareous substance or small bone, adhering 

 to marine bodies. 



Shell. Adhering, irregular, inequivalve, inequilateral, ostrace- 

 ous ; inferior valve a little flatter than the superior, divided at the 

 summit into two sloping branches, whose approxmiation forms a 

 large oval hole ; the superior valve, which is the larger, has an 

 oval excavation under the summit ; a subcentral muscular impres- 



