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. 



GENUS OSTREA. Ltrmcetw. 

 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. 



