CONCHIFEHA. 407 



Family I.— OsTREiDiE. 



Shell inequivalvej slightly inequilateral, free or adherent, 

 resting on one valve ; beaks central, straight ; ligament in- 

 temal ; epidermis thin ; adductor impression single, behind the 

 centre ; pallial line obscure ; hinge usually edentulous. 



yl/irma^ marine; mantle quite open ; very slightly adherent 

 to the edge of the shell ; foot small and byssiferous, or obsolete ; 

 gills crescent- shaped, 2 on each side ; adductor muscle composed 

 of two elements, but representing only the posterior shell-muscle 

 of other bivalves. 



The union of the Ostreidce and PecUnidce, as proposed by the 

 authors of the " History of British Mollusca," has not proved 

 satisfactory. The genus Ostrea stands quite alone, and distinct 

 from all the PecUnidce. in the structure of its gills, which are 

 like those of Avicula, and by resting on its left valve. The shell 

 also is more nacreous than that of the scallops 



OsTEEA, L. — Oyster. 



Synonyms, Amphidonta and Pycnodonta, Fischer. Peloris, 

 Poll. 



Type, 0. edulis, L. 



Example, 0. diluviana, PL XVI., Fig. 1. 



Shell irregular, attached by the left valve ; upper valve flat 

 or concave, often plain ; lower convex, often plaited or 

 foliaceous, and with a prominent beak; ligamental cavity 

 triangular or elongated ; hinge toothless ; structure sub- 

 nacreous, laminated, with prismatic cellular substance between 

 the margins of the laminse. 



Animal with the mantle-margin double, finely fringed ; gills 

 nearly equal, united posteriorly to each other and the mantle- 

 lobes, forming a complete branchial chamber ; lips plain ; palpi 

 triangular, attached; sexes distinct.* 



Distribution, 70 species. Tropical and temperate seas. Norway, 

 Black Sea, &c. 



Fossil, 200 species. Garb. — . United States, Europe, India. 



The interior of recent oyster- shells has a slightly nacreous 

 lustre ; in fossil specimens an irregular cellular structure is 

 often very apparent on decomposed or fractured surfaces. Fossil 

 oysters which have grown upon Ammonites, Trigonice, &c., 

 frequently take the form of those shells. 



In the " cock's-comb " oysters both valves are plaited; 0. 

 diluviana sends out long root-like processes from its lower 



* The course of the alimentarj' canal in the common oyster is incorrectly repre- 

 sented by Poll, and copied in the Crochard ed. of Cuvier. 



