OSTREID^. 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 Ostreidfe and Pectinidie, 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 Pectinidse 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. 



Ostrea, Linn. 



Syn. — Peloris, Poll. 



Distr. — 70. sp. Tropical and temperate seas. Norway, 

 Black Sea, etc. Fossil, 200 sp. Carb. — ; United States, Europe, 

 India. 0. Virginica^ Linn. (vol. i, t. 22 ; cxxxiii, 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 laminae. 



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

 nearl}^ equal, united posteriori}^ to each other and the mantle- 

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

 triangular, attached ; sexes distinct. 



The interior of recent oj^ster-shells has a slightly nacreous 

 lustre ; in fossil specimens an irregular cellular structure is 

 often ver^' apparent on decomposed or fractured surfaces. Fossil 

 oysters which have gi'own upon Ammonites, Trigonine, 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 

 (0. longirostris) attains a length of two feet; 0. Talienwan- 

 ensis, Crosse, grows to the length of three feet in the Ba}^ of 

 Taichou, Japan. The greatest enemy of oystei'-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 b^^ the 

 left valve ; umbones subspiral, turned to tlie posterior side (i. e. 

 reversed) ; right valve opercular. 0. Hianboldtii, Fischer 



20 



