408 MANUAL OF THE MOLLUSC A. 



valve. The "tree-oyster" {Dendrcatrea, 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. 

 From 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 confervm in the tanks. 

 Sub-genera. Gryphcea, Lamarck. 

 Gr. 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. 

 Fig. 215. GrypJuia. Lias — Chalk. Europc, India. 



Exogyra, Sby. E. conica, PI. XVL, 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. 

 Diniya (Deshaycsana), 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. Achsous, PL XVL, Fig. 3. 



Synonyms, Fenestrella, Bolten; Cepa, Humph. Aenigma, 

 Koch. 



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



