B1VALVIA. 29 



Shell thick, orbicular, uneven, not imbricated ; lower valve deep, upper valve nearly 

 flat ; hinge area broad and flat, muscular impression large and orbicular ; both valves 

 coloured within. 



Diameter, 3^ inches. 



Locality. Bracklesham {Dixon). 



" Colour is so rare an occurrence among oysters, and especially among fossils, that we 

 are glad to accept it for a specific mark. The present shell belongs to a section of the 

 genus which contains species whose surfaces are not imbricated, but covered by a con- 

 tinuous plate of a fibrous structure ; in this the structure is, however, obscure ; in O. 

 tabulata and 0. dor sat a, &'c, it is very easily detected. Old shells seem to be imbricated 

 because the edges of the lamina? are worn away. The surface of the young shell, well 

 shown in an individual which has been attached to a large Nautilus, is nearly smooth, 

 but irregularly marked with distant, short, interrupted striae ; the hinge area projects into 

 the cavity of the shell." — Sowerby. 



This species is rare, and colour appears to be its most distinguishing character. 

 The upper valve exhibits some depressed, broad, irregular, and rather obscure radia- 

 tions, separated by a small depressed line and coloured rays ; the lower valve shows 

 a broad mark of attachment ; and the shell is somewhat compact, with fine lines of growth 

 and small or incipient lamella?, but it is less laminated than yir/antea, which it otherwise 

 much resembles; the muscle-mark is transversely rounded, and of the form usually assumed 

 in the orbicular oysters. 



17. Ostrea prona, S. Wood. Tab. Ill, fig. 3, a, b. 



Spec. Char. 0. testa crassd, obliqud, ctmeatd ; valvd inferiore iumidd, inflatd, profundi' 

 plicatd, plicis radiantibus Jtinc hide fur catis, elevatis, anyulatis ; valvd superior e planatd ; 

 cardine introrsum recurvo. 



Shell thick, strong, oblique, and wedge-shaped ; lower valve tumid, inflated, and deeply 

 plicated ; folds radiating, elevated, angulated, and bifurcated ; upper valve flat, plain ; umbo 

 inflexed. 



Diameter, 2f inches. 



Localities. Brockenhurst, Lyndhurst {Edwards). 



This shell is by no means rare in Mr. Edwards' cabinet. The young of this species 

 strongly resembles 0. fabellula, and like other species in several genera, the immature shell 

 can scarcely be distinguished from some proximate species of less magnitude. I think, how- 

 ever, there is sufficient difference in the full-grown individual to justify its specific separa- 

 tion. The ribs or folds in this species are perfectly angular in well-preserved specimens, 

 while mflabellula they are obtuse or rounded ; this difference is its principal distinction. 



