On the Economical Uses of some species of Testacea. 61 



ents. The Byssus is of a dark green color and metallic hue, and 

 they can move slightly by contracting or extending the muscle to 

 which it is attached. The ancients supposed them to be endowed 

 with peculiar powers of locomotion, describing them as grazing at 

 the bottom of the ocean, with a leader to direct them, Sec* but 

 whether they have any such power is extremely doubtful, at least 

 after attaining the thick shell.f The sexual differences have not 

 hitherto been discovered, although the natives of Ceylon pretend to 

 distinguish the sexes, by the appearance of the shell. Those that 

 are large and flat, they call males; those that are thick, concave and 

 vaulted, they call females ; but Mr. Le Beck, who appears to have 

 carefully examined and dissected this animal, declares he was una- 

 ble to discover any difference. J 



Like the Ostrea edulis, this fish appears to thrive best In a mix- 

 ture of fresh and salt water. Pearls are always the most beautiful 

 in those places of the sea where a quantity of fresh water falls, as at 

 the mouth of rivers and streams,^ while those produced by the shells 

 growing on rocky bottoms, are found to be of a better water than 

 those that lie among sand and coral. 



Of the many suppositions as to the cause of pearls, that of RaU" 

 meur is the most probable, and in the present day generally adopt- 

 ed. He supposed them to be owing to a disease in the fish, as cal- 

 culi in mammalia, and to arise from a ruptured or morbid state of the 

 vessels provided for the secretion of the materials of the shell : most 

 experiments and observations go to prove the truth of it. || 



* Pliny, IX. 35. 



t Mr. Montgomery Martyn makes the following remarks on this shell, but from 

 his very obvious ignorance of natural history, and the general inaccuracy and 

 haste of his observations in this department, they are very little to be relied on. 

 We however give them, as from a modern and widely circulated work, and not 

 entirely without its merits, although abounding in faults. "At certain seasons, 

 the young oysters are seen floating in masses, and are caiTied by the currents 

 round the coasts (of Ceylon.) They afterwards settle and attach themselves by a 

 fibre or beard on coral rocks, and on sand; they adhere together in clusters; when 

 full grown, they again separate, and become locomotive. The pearls enlarge dur- 

 ing six years, and the oyster is supposed to die after seven years." — Martyn 's His- 

 tory of the British Colonies, 2d ed. I. 522, note. 



t Asiatic Researches, ^it stipra. Dr. Kirtland remarks the same with respect to 

 those species of the family of Naiades of Lamarck, which are found in the waters 

 of this country, and he is "persuaded that each sex possesses a peculiar organiza- 

 tion of body, associated with a corresponding form of the shell, sufficiently well 

 marked to distinguish it from the other." — Silliman's American Journal, Vol. 

 XXVI, pp. 118, 119. 



IT Bruce's Travels to discover the Sources of the Nile, VII. 322. 



II Transactions of the French Academy. 



