PELECYPODA. 43 



(Meleagrina and Malleus). Some species of Pinna attain to a length 

 of two feet. They are found imbedded in the sand with the narrow 

 pointed end downwards. They form a large silky byssus, which 

 can be woven or knitted into gloves, socks, &c. (see Table-case H). 

 The u Hammer-Oyster " {Malleus) is so called from its rude resem- 

 blance to a hammer. The " Pearl-Oysters " (Meleayrina margariti- 

 fera) possess rather heavy strong shells, lined with very thick layers 

 of "niother-o'-pearl." Hundreds of tons of these shells are annually 

 collected at the great pearl-fisheries of Ceylon, the Persian Gulf, and 

 West Australia, and imported into Europe. The round pearls, which 

 are valued so highly, are either excrescences of the pearly layer or 

 are found loose in the fleshy parts of the animal. Some small 

 foreign body which has accidentally 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 

 effort of the animal to get rid of the irritation caused by a foreign 

 substance between its valves, by covering it over with nacre, and 

 thus converting it into a pearl, is strikingly illustrated by two 

 specimens in which, in the one case, an entire fish, and in the other 

 a small crab, have been so enclosed (see Table-case F at the end 

 of the Gallery) . 



One of the most ancient and, at the present day, the most impor- 

 tant of the pearl-fisheries is that carried on on the western shores 

 of Ceylon. " The Banks," or spots on which the oysters grow, are 

 at an average depth of 30 to 60 feet, and extend several miles 

 along the coast. The oysters, which should be six or seven years 

 old when collected, are gathered in baskets by native divers aud 

 hauled up by ropes into hundreds of small boats. The shells are 

 then brought to land and placed upon the ground to die and 

 putrefy, and then minutely examined for the pearls, which are either 

 found loose in the shells or imbedded in the fleshy parts of the 

 oysters. As many as two million oysters have been brought ashore 

 on one day ; but the number obtained varies very much according 

 to the state of the banks. A small proportion of the oysters 

 contain pearls; in some only very small ones (seed- or dust- 

 pearls as they are called) are found, and very few contain pearls 



E 



