BIVALVIA. 17 
The principal distinction is the thinness of the shell and the smallness of the hinge- 
area, with rather small and distant ribs in which characters, as well as in the form of 
the muscle-mark, it seems to differ from the young of O. Bel/ovacina, to which it other- 
wise somewhat approaches. I have not seen the upper valve. 
3. Ostrea Betnovacina, Lamarck. ab. III, fig. 1, a, 0, and Tab. VII, fig. 3, a—e. 
Ostres Bettovacina. Burtin. Orvet. de Brux., pl. 10, figs. a—d, 1784. 
— _ Lam. An, du Mus., t. viii, p. 159; and t. xiv, pl. 25, fi 
1806. 
— — J. Sowerby. Min. Conch., t. 388, figs. 1, 2, 1823. 
pl. 49, figs. 1, 2. 
~e — Goldf. Petr. German., t. 11, p. 15, pl. 77, fig. 2. 
— — Nyst. Rech. Coq. Foss. de Hoesselt and KI. Sp., p. 16, No. 41, 
1836. 
— — Nyst. Coq. Foss. Belg., p. 318, pl. 30, fig. 1, a@; pl. 31, fig. 1, 4; 
pl. 32; fig. 1, a; pl. 33, fig. 1, 6; 1843. 
EDULINA, Lam. Hist. des An. sans Vert., t. vi, p. 218, 1822. 
_ — J. Sowerby. Min. Conch., t. 388, figs. 3, 4, 1823. 
—  unpbuLATa? J. Sow. (non Myst). Min. Conch., t. 238, fig. 2, 1819. 
Spec. Char. O. testa ovatd, depressiusculd, valved inferiore convead, radiatin et rugose 
costatd, et syuamoso-foliaced; valvd superiore planatd, obsolete radiatd ; concentrice lamellata ; 
impressione musculari mediocri, ovato-semilunari ; umbonibus subequalibus. 
Shell ovate, rather depressed, lower valve convex, with rugose, radiating ribs or ridges, 
foliations squamose; upper valve nearly flat, and obsoletely ridged by lamellated lines 
of growth; muscular impression of a moderate size, ovately lunate ; umbones nearly 
equal. 
Longest diameter, 4 inches. 
Localities. Charlton, var. a, Dulwich, var. B (Edwards). 
France, Hauteville, Beauvais (Lamarck). 
Belgium, Kleyn-Spauwen, Hoesselt et Lethen (Vys/). 
Oysters occur almost throughout the whole extent of the lower beds of the Eocene 
Formation in England, and all the specimens that have been found were for a long time 
considered by geologists as varieties of O. Bellovacina, and the geographical range assigned 
to this species extends from Clarendon, Pebble Hill, and Newbury, through Reading, to 
Northaw and Rochester, including within these outskirts the central portions of Woolwich, 
New Cross, &c. On expressing an opinion to my friend, Mr. Prestwich, that there were 
two species found in these lower beds, and my wish? if possible, to have them separated, 
and their proper localities assigned, and requesting his assistance for that object, I learnt 
from him that he had, since the publication of his paper, strongly suspected that the 
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