ti 



133 







DISCINA. 

 See G. Orbicula and G. Crania. 



Lamarck has named this singular shell Discina, from 

 each valve presenting, near the centre, a remarkable 

 and very distinct orbicular disk ; that on the superior or 

 upper valve is smooth and pierced, and exhibits in the 

 centre a small papillary elevated summit, giving it the 

 appearance of a Patella : it is also surrounded by a 

 margin longitudinally marked with delicate radiating 

 striee. The disk of the lower margin is very white, 

 obliquely crossed by a notch or cleft, open on both sides. 

 Although the valves of the shell are of an equal size, 

 they are not exactly similar, the upper one being ra- 

 ther more convex, and the lower without strise round 

 its disk. No traces of hinge, ligament, or muscular im- 

 pressions, are visible. It is found affixed to stones on 

 the British coast. This genus, Sowerby remarks, should 

 be entirely expunged, Lamarck having constituted it, 

 without mature consideration, from examples of the 

 Orbicula Norwegica. 



Discina ostreoides. 



