292 THE ZOOLOftlST. 



each case weighing from 270 to 300 pounds, and are ready for 

 shipment. No systematic effort has yet been made to collect 

 pearls at Torres Strait, and such as are found become the 

 property of the men, who secrete them in various ways, often by 

 swallowing them. Some very fine specimens of pearls about the 

 size of a hazel-nut, and of remarkable beauty and clearness, have 

 recently found their way to the market from Torres Strait. 

 Other specimens of a much larger size have been found there, 

 but they were imperfect in shape and colour. 



Formation of Pearls. — In oysters aged four years— which 

 are judged by the shells, weight, and appearance — the best pearls 

 are found. The shell, like the pearl, is formed by the secretion 

 of the animal, and is composed of animal matter and lime. The 

 iridescent hues on the inside of the shell are occasioned by the 

 edges of the thin, wavy, concentric layers overlapping one another 

 and reflecting the light. The minute furrows, containing trans- 

 lucent carbonate of lime, produce a series of more or less brilliant 

 colours, according to the angle at which the light falls upon them. 

 Occasionally some of the finest pearls are found loose in the shell. 

 As many as one hundred pearls have been found in one oyster, 

 but of little or no value. The pearls of the young oyster are 

 yellow, and in the older oyster are of a pinkish hue. 



The Use of Pearl-shells. — The pearl-shells shipped from 

 Australia to the United States and Europe are used principally 

 for the manufacture of knife-handles, shirt-buttons, &c. Con- 

 siderable quantities are also used for papier-mache and other 

 ornamental work. The pearl buttons, shirt-studs, &c., now made 

 in the United States are said to be the best and cheapest in the 

 world — a fact due in great measure to the care used in selecting 

 the material and to the improved methods of cutting. 



NOTES AND QUERIES. 



To purify Water in an Aquarium. — In fresh-water aquaria the 

 introduction of a few pond-snails (such as Planorbis corneus, Pcdudina 

 vivipara, Limnaia stagnalis, L. aiiricularia, &c.), \vhich scour the inner 

 surface of the glass, is tolerably effective ; but a better plan is suggested 

 by a writer in the ' Norsk Fiskeritidende,' who recommends that to every 

 100 grammes of water there should be added 4 drops of a solution of 

 1 gramme of salicylic acid in 300 grammes of water. A gramme is equal 



