90 CONTRIBUTIONS 
internal ribs radiate from the beak, and are more numer- 
ous, and independent of the external ones, which vary in 
number from six to twelve. These internal ribs are dis-° 
posed to terminate, in the left valve with a tubercle—in the 
right with a corresponding depression. In young indivi- 
duals the valves are thin. In all my specimens, the cicatrix 
differs in colour from the rest of the disk, being whitish. 
Three species only of Plicatula have been observed in 
Great Britain, the highest of these in the Chalk Marl 
(Crai Tufau), being of the Cretaceous Group. Lamarck 
describes six. In M. Deshayes’s Tertiary Tables we find 
_ seven, three being from the Paris basin. Mr Say has from 
the Tertiary of Maryland described one species, marginata. 
FAMILY OSTRACEA. ihe 
GENUS OSTREA.  Linneus. 
O. semilunata. Plate 3. Fig. 69. 
Description. Shell semi-lunate, anterior margin much 
incurved ; superior valve incurved—inferior one very con- 
vex ; beaks produced, rounded and flattened ; basal mar- 
gin rounded. 
Diam. .8, Length 2.3, Breadth 1.6, of an inch. 
