BIVALVIA. 27 



Dim ciix'tons, 4 inches by 2. 



Localities. Hempstead {Edwards), Bracklesham {Dixon). 



France, Montmartre, Sceaux, Longjumeau {Deshayes). 



There are two or three species in this genus which have an extended liganicntal area, 

 and arc 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 {0. crassa, ' Chemn./ vol. viii, p. 10, t. 74, f. G78), 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. longirostris as a fossil from Syracuse, but he does not 

 give a figure ; he quotes Goldfnss, as well as Deshayes. 



14. Ostuea marginidentata, S. Wood. Tab. V, fig. 2, a — d. 



Ostrea radiosa. J. Sow., in Dixon's Geol. of Sussex, p. 1/4, 1850. 

 — — Morris. Catal. Brit. Foss., p. 175, 1854. 



Spec. Char. 0. testa ovatd vel orbiculatd, crassa ; valvd inferiore plicatd, plicis squa- 

 mosis, radiantibus ; valvd superiore plana ; marginibus valde crenulatis vel denticulatis ; 

 hnpressione musculari magna. 



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, 3^ inches. 



Localitg. Bracklesham {Edwards.) 



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. externa, 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. 



* I have retained the name of longirostris for this species, as it is presumed to be identical with the 

 Paris Basin shell, which was the one originally so called. 



