254 MANUAL OF THE MOLLUSC A. 



of the shell ; foot small and byssiferous, or obsolete ; gills crescent-shaped, 

 2 on each side ; adductor musele composed of two elements, but representing 

 only the posterior shell -muscle of other bivalves. 



OSTREA, L. Oyster. 



Syn. Amphidonta and Pycnodonta, Fischer. Peloris, Poli. 



Type, O. edulis, L. Ex. 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 laminae. 



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

 equal, united posteriorly to each other and the mantle-lobes, forming a com- 

 plete branchial chamber; lips plain; palpi triangular, attached; sexes 

 distinct.* 



Distr. 60 sp. Tropical and temperate seas. Norway, Black Sea, &c. 



Fossil, 200 sp. Carb. . U. 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 de- 

 composed or fractured surfaces. Fossil oysters which have grown upon 

 Ammonites, Trigoniee, &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 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 

 (0. longirostris] attains a length of two feet. The greatest enemy of oyster- 

 banks is a sponge (Cliona), which eats into the valves, both of dead and living 

 shells; at first only small round holes, at irregular intervals, and often dis- 

 posed in regular patterns, are visible ; but ultimately the shell is completely 

 mined and falls to pieces. Natural oyster-banks usually occur in water 

 several fathoms deep ; the oysters spawn in May and June, and the fry 

 ("spats") are extensively collected and removed to artificial grounds, or 

 tanks, where the water is very shallow ; they are then called " natives," and 

 do not attain their full growth in less than 5 or 7 years, whilst the " sea- 

 oysters" are full-grown in 4 years. Native oysters do not breed freely, and 

 many sometimes die in the spawning season; they are also liable to be killed 

 by frost. The season is from August 4 to May 12. From 20 to 30,000 

 bushels of " natives" and 100,000 bushels of sea-oysters are annually sent 

 to the London market. Many other species of oysters are eaten in India, 

 China, Australia, &c. " Green oysters" are those which have fed on con- 



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

 sented by Poli, and copied in the Crochaid ed. of Cuvier. 



