254 MANUAL OF THE MOLLUSCA. 



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. 



OsTREA, L. Oyster. 



S^n. Amphidonta and Pycnodonta, Fischer. Peloris, Poli. 



I^pe, O. edulis, L. Ex. 0. diluviana, PI. 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.* 



Bistr. 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, 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 fi'om 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. long'irostris) 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 Crochard ed. of Cuvier. 



