Gapers^ Piddocks^ and Ship-worms 



^71 



or auricle. In the present .species this auricle is very 

 large, and the hinge-plate is very broad and thick, 

 with a knob and a tooth in the centre. The sheath, 

 too, is shorter, and often absent. 



Several other species of foreign origin are fre- 

 quently found in drift-wood on our shores. 



The animals comprised in the sub-order Anatinacea 

 have a single gill on each side, the outer fold of which 

 is prolonged over the back ; the mantle margins 

 united almost throughout ; the siphons more or less 

 united and their orifices fringed. The shells are thin, 

 the valves often unequal, usually coated with nacre 

 ^^'ithin. The impressions of the mantle and muscles 

 are not well marked. 



The Pandora-shell {Pandora incequivalvis) not only 

 has the valves unequal, they are inequilateral also. 

 They gape behind where the short 

 siphons protrude. It will be seen in 

 the figure that these siphons are 

 united almost to their full length, 

 yet diverge considerably from each 

 other. The left valve is flat, the right convex and 

 overlapping the left. They are of variable thickness, 

 glossy without and of a pearly 

 white colour. The horn-coloured 

 cartilage reposes in a groove of 

 each valve. The hinge-plate of 

 the left valve bears a rib which 

 fits into a corresponding furrow 

 in the other valve. There is one 

 cardinal tooth in each valve, that of the right valve 

 being erect, that of the left horizontal. The Channel 

 Islands, Weymouth, and Studland are the only British 



Pandora-shell 



Left valve of Pandora 



