CONCHIFERA. 407 
FAmMILty I.—OsTREIDA. 
Shell inequivalvye, slightly inequilateral, free or adherent, 
resting on one yalye; beaks central, straight; ligament in- 
ternal; epidermis thin ; adductor impression single, behind the 
centre; pallial line obscure; hinge usually edentulous. 
Animal 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 Ostreide and Pectinide, 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 Pectinide in the structure of its gills, which are 
like those of Avicula, and by resting on its left valye. The shell 
also is more nacreous than that of the scallops 
OsTREA, L.—OYSTER. 
Synonyms, Amphidonta and Pycnodonta, Fischer. Peloris, 
Poli. 
Type, O. edulis, L. 
Example, O. 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 laminz. 
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. Carb. —. United States, Kurope, 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 yalves are plaited; O. 
diluviana sends out long root-like processes from its lower 
* The course of the alimentary canal in the common oyster is incorrectly repre- 
sented by Poli, and copied in the Crochard ed. of Cuvier. 
