While this species is gregarious, it is not located in densely covered beds, but is scattered in patches over the reefs. Some of these are miles in length and contain scores of tons, but visually they are very much smaller. The oysters occur principally on rocky bottom, and also on clay and sand when well covered with seaweeds, but are rarely found on muddy ground. They are most numerous in the channels where the current is strong. The small oysters are generally loosely attached by the byssus to rock, gravel or other shells; while the mature ones lie loosely on the bottom or slightly turned in the sand. The second species of the Australian pearl-oysters, _Margaritifera margaritifera_, is smaller, rarely exceeding eight inches in diameter and a weight of two pounds. The distinguishing characteristic is the black edge bordering the inner surface of the shell, whence it acquired the local designation “black lip.” This variety is not rare in Queensland, and in Western Australia its range extends as far as Champion Bay in Lat. 29° S. However, the catch is small compared with that of the _Margaritifera maxima_, amounting to only two or three per cent. in Queensland. In 1905, the export of “silver lip” and “golden-edged” from Thursday Island was 527 tons, and of “black lip” only 11 tons; in 1904, these figures were 778 and 7 respectively. In Western Australia the percentage of yield is much larger than this. The third species, _Margaritifera carcharium_, is confined almost entirely to the limits of Sharks Bay, on the extreme western coast of Australia. At maturity it is the smallest of the three, averaging three or four inches in diameter, and about equals in size the Lingah pearl-oyster of the Persian Gulf. The percentage of pearls therefrom is relatively greater than from the larger variety; but, owing to its small size and lack of thickness, the shell is of little commercial value. The value of the output in recent years has approximated two or three thousand pounds sterling, which is very much less than formerly, the value of the shell having greatly decreased since the introduction of the Mississippi shell in button manufacture.