362 THE OCEAN WORLD. 



themselves as parasites in the shell of the mussel ; to the spawn of 

 star-fishes or medusae that the mussel may have swallowed. But, 

 probably, the true cause of this kind of poisoning resides in the pre- 

 disposition of individuals. The remedy is very simple : an emetic, 

 accompanied by drinking plentifully of slightly acidulated beverages. 

 We have now come to the twentieth family, the Aviculidse, which 

 contains Avicula, Malleus, Meleagrina, Perna, and Pinna. The shells 

 of the sub-genus Malleus (hammer-headed oysters) have a rough re- 

 semblance to the implement from which they derive their name. The 





Fig. i6i.-Malleusvulgari S (Lamarck). 



valves are nearly equal, blackish, and somewhat wrinkled on the 

 exterior, often brilliantly nacred in the interior. They are enlarged 

 to the right and left of the hinge, forming prolongations on each 

 side, which give them the fancied resemblance of a hammer-head 

 (Malleus vulgaris, Fig. i6i>. At the same time they grow in a 

 direction opposite to the hinge, which gives an appearance something 

 approaching the handle of the implement. 



This is the first feature which a glance at Malleus alba (Fig. 162) 

 conveys. The hinge is without teeth, having instead a deep conical 

 fossette or dimple, for the reception of a very strong ligament, which 

 acts upon the valves. The animal is contained in the interior 



