14 MOLLUSCA FROM THE CRAG. 
OstREA DENTICULATA. Desh. 2d ed. Lam., tom. vii, p. 225. 
— — Broc. Conch. Foss. Subap., p. 568, 1814. 
—  routtosa. Broce. Conch. Foss. Subap., p. 563, 1814. 
— vuneautata. Nyst. Coq. Foss. de Belg., p. 325, pl. 24, fig. 8; pl. 26, fig. 8; and 
pl. 34, fig. 1, a, a, 6, 1844. 
—  psorEALIS? Desh. 2d ed. Lam., tom. vii, p. 220. 
_— -- Gould. Inv. Massach., p. 137, 1841. 
— spectrum. (Leathes, M.S.) Woodward Synop. Tab. Brit. Org. Rem., p. 20, 1830. 
— — S. Wood. Catalogue, 1840. 
—  waMeELLosa. Goldf. Pet. Germ., vol. 11, p 18, t. 78, fig. 3, a, 6. 
OstREUM VULGARE. Dacosta. Brit. Conch., p. 154, pl. 11, fig. 6, 1778. 
Avuricutarta MAxIMA. Dale. Hist. and Antiq. of Harwich, p. 291, t. xii, fig. 2, 1730. 
Ency. Method., pl. 183, figs. 3, 4? pl. 184, figs. 7, 8. 
Spec. Char. Testa variabile, plerumque ovato-rotundatd, basi subattenuatd, concen- 
trice lamellosd, interdum radiato-costatd, costis imbricatis, undulatis ; valvd superiori 
pland. 
Shell variable, for the most part of a roundedly ovate form; base of the shell, 
slightly attenuated, concentrically lamellated, sometimes costated with imbricated and 
undulated ribs ; upper valve flat. 
Diameter, 3 inches. 
Locality. Cor. Crag, Ramsholt, Sudbourn, Gedgrave. 
Red Crag, Passim. 
Recent, Britain. North America? Mediterranean ? 
The earliest appearance of the true edible Oyster seems to have been in the 
Coralline Crag Period, so far as it is possible to determine a species in this truly 
variable genus, and its diversified forms were then as great as we see them in the 
recent shell of the present seas. In the same deposit, at Ramsholt, and in the same 
bed in close proximity, are two very distinct varieties, from which, as well as from a 
great dissimilarity of form, solidity of shell, and other differences, it was presumed at 
the time my Catalogue was drawn up, that they were specifically distinct; but the 
variety then considered different, and which passed under the provisional name of 
spectrum, corresponds with the recent shell that is now determined by British Con- 
chologists to be only a local variation, and they are, therefore, here united into one 
species. At Ramsholt there is a complete bed of this shell (parasitica), sometimes 
attached to each other in clusters, or often to the large species of Balanus, so abun- 
dant in that locality ; at this place, also, the thick and ponderous variety (fig. 1, @), is 
occasionally met with, but not in any great profusion; the latter shell is at this place 
more isolated in its habits and regular in form, and is sometimes marked with nearly 
obsolete radiating costa, with a very rugose exterior to the lower or adherent valve, 
while in the upper valve there are no markings, except the regular lines of growth ; 
in the other variety, there is less appearance of the radiating ribs with a more 
lamellated exterior, and the shell is much thinner, and less regular in shape, partaking 
