362 THE OCEAN WORLD. 
themselves as parasites in the shell of the mussel; to the spawn of 
star-fishes or medusz 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 Aviculide, 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 theirname. The 
Fig. 161.—Malleus vulgaris (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. 161). 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 Aalleus 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 
