BIVALVIA. 27 
Dimensions, 4 inches by 2. 
Localities. Wempstead (Ldwards), Bracklesham (Dixon). 
France, Montmartre, Sceaux, Longjumeau (Deshayes). 
There are two or three species in this genus which have an extended ligamental area, 
and are equally deserving of the present or a similar appellation.* ‘The fossil oyster from 
Lisbon has the hinge area as long as that of the Eocene species, and it has also been called 
longirostris, but the shape of the shell is very different. ‘The recent American oyster 
O. Virginica (O. crassa, ‘Chemn.,’ vol. viii, p. 40, t. 74, f. 678), has a similar character, 
but is, I think, distinct. 
The present species does not appear to have been abundant in the British Eocene 
Seas, and I have not seen the upper valve; M. Deshayes speaks of the French shell as by 
no means rare, and he gives four distinct varieties of his species. Philippi (‘En. Moll. 
Sic.,’ vol. ii, p. 64) introduces O. dongirostris as a fossil from Syracuse, but he does not 
give a figure; he quotes Goldfuss, as well as Deshayes. 
14, Ostrua MARGINIDENTATA, 8S. Wood. Tab. V, fig. 2, a—d. 
Ostrea RADIOSA. J. Sow., in Dixon’s Geol. of Sussex, p. 174, 1850. 
— — Morris. Catal. Brit. Foss., p. 175, 1854. 
Spec. Char. O. testd ovatd vel orbiculatd, crassa ; valvd inferiore plicald, plicis squa- 
mosis, radiantibus ; valvd superiore pland ; marginibus valdé crenulatis vel denticulatis ; 
impressione musculart magni. 
Shell ovate or orbicular, thick, and strong; lower valve plicated, with radiating and 
rough ridges or ribs; upper valve flat and plain, margin crenulated, muscular impression 
large and slightly curved or reniform. 
Diameter, 34 inches. 
Locality. Bracklesham (Adwards.) 
This is by no means rare. Among Mr. Edwards’ specimens there is great variation. 
Fig. 2, d, resembles the form and most of the characters of O. extensa, as given 
by M. Deshayes. Many specimens from Bracklesham have adhered by a large surface, 
extending over nearly the whole valve ; in these cases the interior is shallow, and the shell 
is more orbicular ; when the adherence is by a small portion of the surface or by the beak 
only, the valves are then more elevated and deeper. I have given a view of the interior 
of both valves, to show the difference in form of the adductor muscle-mark. In the 
specimen, fig. 2, a, the animal has extended the shell into a sort of shoulder; and the 
adductor muscle has followed the course taken by the mantle; from this distortion the 
muscle-mark is much altered. 
* [ have retained the name of Jongirostris for this species, as it is presumed to be identical with the 
Paris Basin shell, which was the one originally so called. 
