I HE PEARL OYSTER. 



237 



Few of the bivalves have been esteemed " fancy " shells or commanded high prices, but some 

 of the Chamas and Spondyli are very beautiful, and might well distract a Dutch or French collector. 

 Sowerby valued Spondylus regius, in the Tankerville collection, at .25, the best Chama at '3 3s., 

 and Isocardia at 8 8s. (!) ; sEtheria elliptica at 21 (!) ; and the Lucina childreni, now in the British 

 Museum, at 10 10s., because it had the hinge reversed. Mr. Norris gave '20 for a Miilleria, 

 an extraordinary shell to the conohologist (although most unattractive to the eye), of which M. 

 D'Orbigny sent several specimens to the British Museum in exchange for a fossil Pentacrinus. 



The genus Plicatula has an irregular shell, with plicated valves, and is fixed to some foreign 

 body by the beak of the right valve. The Plicatulw are tropical shells. Ten species occur in the 

 East and West Indies, the Philippines, &c. 



FAMILY IV. AVICULIDJE. 



The shell is very oblique and the valves unequal ; it is attached by a byssus ; they are pearly 

 within, the outer layer is cellular, the lobes of the mantle are fringed at the mar-gin ; it has two 

 gills on each side. The Pearl Oysters, or " Wing-shells," as they are called, are mostly tropical. 



Genus Avicula. The shell is very unequal ; there is a fold for the byssus in the right valve, 

 beneath the ear of the shell ; there are one or two small teeth in the hinge ; the valves are obliquely 

 oval. Twenty-five species occur living in about twenty-five fathoms water in Britain, the Medi- 

 terranean, India, &c. 



Genus Meleagrina. The valves of the Peai-1 Oyster are flattish and nearly equal in size, the gills 

 are equal and crescent-shaped, ___^= ^^^^^^ 



the foot finger-like and grooved. j| > 



Meleagrina is less oblique than 

 the other Aviculidce. They are 

 found living in Madagascar, 

 Ceylon, Swan River, &c. 



The shells of the "Pearl 

 Oyster " afford the substance 

 known as " mother-o'-pearl," so 

 largely employed in the manufac- 

 ture of buttons and for pa pic r- 

 tnuche inlaid-work, &c. 



Prof. T. C. Archer mentions 

 that there are three principal 

 kinds of these mother-o'-pearl shells brought to market at Manila (which was the depot for the 

 Pearl Oyster trade). 



One kind is known as the silver-lipped Pearl Oyster, from the Society Islands ; another 

 the black-lipped variety, from Manila ; the third, from Panama, is smaller than the others. 

 About 250 tons of these shells were annually imported into Liverpool alone. They also yield the 

 " oriental " pearls of commerce. The principal pearl fisheries are in the Persian Gulf and Ceylon. 

 Pearls are produced by many bivalves, but by none in greater perfection than by the Meleagrina 

 margaritifera. They are caused by particles of sand or other foreign substance finding its way into 

 the cavity of the valves, and getting between the animal and its shell ; the irritation causes a 

 deposit of nacre, forming a projection on the interior, generally more brilliant than the rest of the 

 shell. Completely spherical pearls can only be found loose in the muscles or other soft parts of 

 the animal. The Chinese obtain them artificially by introducing into the living Hyria foreign 

 substances, such as pieces of mother-o'-pearl fixed to wires, which thus become coated with a more 

 brilliant material. Similar prominences and concretions pearls which are not pearly are formed 

 inside porcellanous shells. These are as variable in colour as the surface on which they are formed. 

 They are pink in Turbinella and Strombus ; white in Ostrea ; white or glossy, purple or black, in 

 Myt'dus ; rose-coloured and translucent in Pinna. 



The pearl fisheries of the Persian Gulf and Ceylon give employment annually to several hundred 

 boats and many thousand men. The entire amount of revenue derived from the pearl fisheries of 



MELEAGRINA MAKGAIUTIFERA A, OUTSIDE J H, INSIDE OF SHELL. 



