408 



MANUAL OF THE MOLLUSCA. 



valve. The "tree-oyster" {Dendrcstrea, 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 disposed 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 five or seven years, whilst the " sea-oysters " 

 are full-grown in four years. Native oysters do not breed freely, 

 and sometimes many die in the spawning season ; they are also 

 liable to be killed by frost. The season is from August 4 to May 12. 

 I'rom 20,000 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 confervce in the tanks. 

 Sub-genera. Gryphcea, Lamarck. 

 G. incurva, Sby. (section), Fig. 

 215. Free, or very slightly at- 

 tached ; left valve with a promi- 

 nent, incurved umbo ; right valve 

 small, concave. Fossil, 30 species. 

 Lias — Chalk. Europe, India. 

 PI. XVI., Fig. 2. Shell chama- 

 shaped, attached by the left valve ; umbones sub-spiral, turned 

 to the posterior side {i.e. reversed) ; right valve opercular. 

 Fossil, 46 species. L. Oolite — Chalk. United States ; Europe. 

 Dimya (Deshayesana), Eouault, 1859. Mem. Soc. Geol. 

 b. III. 471, t. 15. Fig. 3. L. Eocene, Paris. The figure is most 

 like an oyster, and the " second adductor impression," on 

 account of which it is named Dimya^ is rather like the small 

 anterior scar in Peden (Fig. 210). 



Anomia, L. 



Etymology, anomios, unequal. 

 Example, A. Achaeus, PL XVI., Fig. 3. 

 Synonyms, Fenestrella, Bolten; Cepa, Humph. Aenigmai 

 Koch. 



Shell sub-orbicular, very variable, translucent, and slightl 



rig. 215. Gryphcea. 



Exogyra, Sby. E. conica. 



