19 
OSTREA gregarea. 
TAB. CXI.—Figs. 1 and 3. 
Spec. Cuar. Clustered, oblong, curved, plaited s 
plaits many, rugged, diverging from a longi- 
tudinal plait or sulcus; valves unequally con- 
vex, beaks produced and curved. 
a 
"Tue lower valve is usually more concave than the other, 
and more or less carinated; the general form of the shell is 
very variable, nevertheless it constantly curves towards 
that side in which lies the muscular impression. The 
plaits are often branched near their commencement; they 
vary in length, several not reaching the central line; those 
in the hollow side are smallest. There are no vestiges of 
ears by the sides of the hinge, the pit of which has a 
central concavity in the lower valve, and a corresponding 
convexity iu the other, characters which distinguish this 
from the plaited Oyster found in the green Sand. The shell 
is rather slender, especially towards the edges. 
Immense masses of these shells haye been found near 
Devizes: they are of a greyish colour, and so much at- 
tached to each other, that it is difficult to distinguish the 
valves, and pairs nearly detached are rare, Mr, Sheffield 
has fine specimens, which he was so good as to lend me; 
for others I am indebted to Mrs. Gent. A greyish Lime- 
- stone often fills the insides, and sometimes partially covers 
the outsides. 
