408 MANUAL OF THE MOLLUSCA. 
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 
(O. 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 1s completely mined and 
falls to pieces. Natural oyster-banks usually occur in water 
seyeral 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 isfrom August 4to 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. 
Zam ‘‘ Green oysters” are those which 
“{ have fed on conferve in the tanks. 
\\} Sub-genera. Gryphcea, Lamarck. 
\\ jG. imcurva, Sby. (section), Fig. 
|) 215. Free, or very slightly at- 
/ tached; left valve with a promi- 
nent, incurved umbo; right yalye 
= small, concave. Jossil, 30 species. 
Fig. 215. Gryphea. Lias — Chalk. Europe, India. 
Exogyra, Sby. EH. conica, Pl. 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), Rouault, 1859. Mém. Soc. Géol. 
b. III. 471, t. 15. Fig. 3. LZ. Hocene, 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 Pecten (Fig. 210). 
ANomtIA, L. 
Etymology, anomios, unequal. 
Example, A. Achzeus, Pl. XVI., Fig. 3. 
Synonyms, Fenestrella, Bolten; Cepa, Humph. Aenigma, 
Koch. 
Shell sub-orbicular, yery variable, translucent, and slightly 
