34 
SHELL GALLERY. 
resemblance to a hammer. The " Pearl-Oysters " (Meleagrina mar- 
garitifera, Fig. 29) possess rather heavy strong shells, lined with very 
thick layers of " mother-o'-pearl." Hundreds of tons of these shells 
are annually collected at the great pearl-fisheries of North and West 
Australia, and imported into Europe. The pearl-oyster of Ceylon 
(M.fucata, Case 146) is a smaller species, and collected more for the 
pearls than the shells. The round pearls, which are valued so highly, 
are either excrescences of the pearly layer or are found loose in the 
British " Fan-Mussel " {Pinna pectinata) : a, the byssus. Case 150 
fleshy parts of the animal. Some small foreign body which has acci- 
dentally penetrated under the mantle and irritates the animal is 
covered with successive concentric layers of nacre, thus attaining 
sometimes, but rarely, the size of a small filbert. The nacre is 
generally of the well-known pearly-white colour, very rarely dark, 
and occasionally almost black. The action of the animal in secreting 
successive layers of nacre over any foreign body which intrudes 
between the mantle-folds, and thus converting it into a pearl, is 
strikingly illustrated by two specimens in which, in the one case, an 
