BIVALVIA. 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. 
Dianeter, 3% inches. 
Locality. Bracklesham (Dizon). 
“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 O. dorsata, &e., it is very easily detected. Old shells seem to be imbricated 
because the edges of the Jamin 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 strize ; 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 Jess laminated than yivantea, which it otherwise 
much resembles ; the muscle-mark is transversely rounded, and of the form usually assumed 
in the orbicular oysters. 
17. Osrrua prona, 8S. Wood. ‘Tab. II, fig. 3, a, 6. 
Spec. Char. O. testd crassa, obliqud, cuneata ; valvd inferiore tumidd, inflata, profunde 
plicatd, plicis radiantibus hine inde furcatis, elevatis, angulatis ; valvd superiore planala ; 
cardine tntrorsum recurve. 
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, 2% inches. 
Localities. Brockenhurst, Lyndhurst (Zdwards). 
This shell is by no means rare in Mr. Edwards’ cabinet. ‘The young of this species 
strongly resembles O. flabellula, 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 mdividual to justify its specific separa- 
tion. The ribs or folds in this species are perfectly angular in well-preserved specimens, 
while in fadellula they are obtuse er rounded ; this difference is its principal distinction. 
