1 90 OYSTERS, AND ALL ABOUT THEM. 



It is common in the Mediterranean and Cherbourg. It is 

 distinguishable from the present species by the ribs of the 

 lower valve being angular instead of rounded. 



THE HAMMER-HEADED OYSTER. 



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 resemblance to the 

 implement from which they derive their name. The 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 pro- 

 longations on each side, which give them the fancied 

 resemblance of a hammer-head ( Malleus vulgaris ). 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 

 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 of the shell, its mantle fringed 

 by very small tentacular appendages. Only six actually 

 living species of the genera are known, which are inhabit- 

 ants of the Indian Ocean, of the Australian seas, and the 

 Pacific Ocean, (a) 



(a) "The Ocean World," p. 362. 



