OSTREIDZ. 297 
to the edge of the shell; foot small and byssiferous, or obsolete ; 
gills crescent-shaped, two on each side; adductor muscle com- 
posed 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 Pectinidze 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. 
OstrREA, Linn. 
Syn.—Peloris, Poli. 
Distr.—i0 sp. Tropical and temperate seas. Norway, 
Black Sea, ete. Fossil,200 sp. Carb.—; United States, Europe, 
India. O. Virginica, Linn. (vol. i, t. 22; exxxili, 34). 
Shell irregular, attached by the left valve; upper valve flat 
or concave, often plain; lower convex, often plaited or folia- 
ceous, and with a prominent beak; ligamental cavity triangular 
or elongated; hinge toothless; structure subnacreous, lami- 
nated, with prismatic-cellular substance between the margins of 
the lamine. 
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. 
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, Trigoniz, etc., 
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 
valve. The “ tree-oyster’? (Dendrostrea, Sw.) grows on the 
root of the mangrove. Oyster-shells become very thick with 
age, especially in rough water; the fossil oyster of the Tagus 
(O. longirostris) attains a length of two feet; O. Talienwan- 
ensis, Crosse, grows to the length of three feet in the Bay of 
Taichou, Japan. The greatest enemy of oyster-banks is a 
sponge, which eats into the valves, both of dead and living 
Shells ; at first only small round holes, at irregular intervals, 
and often disposed in regular patterns, are visible; but ulti- 
mately the shell is completely mined and falls to pieces. 
EXOGYRA, Say. (Amphidonta, Fischer. Ceratostreon, Rhyn- 
chostreon, Bayle, 1879.) Shell Chama-shaped, attached by the 
ieft valve; umbones subspiral, turned to the posterior side (7. e. 
reversed); right valve opercular. O. Humboldtii, Fischer 
20 
