388 
THE TROPICAL 
AGRICULTURIST. 
[Dkc. 1, 1900. 
the biological conditions which exist at different 
pearling ceu ties. And tronisuchsourcesaatlie.se 
a great part of the material here dealt with has 
be;n derived.* 
VARIETIES OF PEARL OYSTER.S. 
Pearls are produced by quite a number of 
different molluscs, and some of these are very 
widely distributed. Among the bivalvei^, the 
gTea,b Iridacna, the conimou oyster (Os^ca ec^wfe), 
and many species of Pinna prcduee pearls. 
They are also found in certain species of Unis 
and Anodonta — fresh-water shells— which fur- 
nish the river pearls of Europe and North 
America. Certain univalve shells also produce 
pearls, for example : — Stimnbus gigcis, the 
giant " conch-shell," and several species of Tur- 
binella. These produce the pink peix.rls of the 
Bahamas and of the West Indies generally. 
The ordinary pearls of commerce may be divided, 
roughly, into two classes, viz., false or "seed"' 
pearls, and true or " Oriental" iiearls. The two 
kinds differ greatly both in beauty and value, 
and are produced by two very distinct genera of 
shells. These are (1) Placuna, with a single 
pearl-bearing species, which belongs to the family 
of Anomiid(r ; and (2) Meleagriiia, whicii 
includes several pearl-bearing species, and which 
belongs to the family of Avictilidre. 
Plamtna ijlacciita, commonly knowti as the 
" window-shell" of the Chinese (because its sp mi- 
transparent valves are frequently used in China 
for illuminating dwelling-houses), is the "pearl 
oyster " of the Red Sea, the Persian Gulf, 
Karachi, the Bay of Tamblegaiu near Trinco- 
raaiee, and the Eastern Archipelago generally as 
far as Hongkong. Although it produces only 
" seed " pearls, an immense number of people are 
annually employed in its collection. The pearls 
themselves are seldom of much value, though 
there is generally a demand for them in the 
markets of the East. They are, however, of little 
importance in the present inquiry ; and so we 
need not further consider them here. 
TRUE PE/VKL OYSTERS. 
All of the species of true pearl oysters belong, 
as 1 have just said, to the family of Aviculidoi— or 
"wing-shells— and with this family it will now be 
necessary foriis to form some acquaintance. In the 
first place, it is one of most respectable antiquity. 
No less than three hundred species have been 
recorded in the fossil state, some even from the 
lower Silurian strata. As a large number of these 
fossil forms have been discovered in northern 
latitudes, it is probable that at one time in the 
earth's history the family was very widely distri- 
buted, though it is now almost entirely restricted 
to tropical and temperate seas. At present only 
twenty-five recent species are known. Of this 
number, live, which belongs to the genus 3Ielea- 
gri/ia, are now fished for pearls. These consii- 
tute the pearl oysters of Madagascar, Ceylon, 
* I take this opportunity of expressing my thanks to 
Mr. R Etheridge, the Director of the Australian 
Museum, Sydney, N.S.W., and to Mr. Gharles Hedley, 
the Conchologiet of that Institution, for valuable 
reports and statistics. Also to Professor H A Pillsbury, 
of the Academy of Sciences. Philadelphia, U.S.A., and 
Mr. 1 Ni.shigawa, of the Imperial Fisheries Bureau, 
Tokyo.Japan, for information kindly supplied. Further, 
I am mrler a special obligation to Mr. -John Ferguson, 
o£ Colombo, who has most kindly placed at ray dis- 
posal a/valnablo collectlpu of Pdf^rs >:elatiDg to various 
jiearl fisheriea. 
Japan, the South Sea Islands, Panama, California, 
and the northern and western Coasts of Australia. 
Zoologically speaking, they do not belong to the 
true oy.--tHrs (Uxfreidce), but are ranch more 
nearlj' allied to the niussels, being furnished with 
a byssLis, or "cable," by which they are in the 
habit of mooring themselves to rocks and other 
substances at the bottom of the sea. 
The classification of the different species 
(and vnrieties) of Melemirina has not yet 
been .systematically worked out ; conse- 
quently, llieir correct identification is a 
matter of cou.siderabie difficulty. We find, lor 
instance, quite a numbei of .shells — bearing both 
jjearls and mothei-of-pearl — designated by the com- 
mon name of M. in<trgaritifera ; whereas, in all 
probability, they belong to very distinct s|jecies. 
True, M. marijuritifciu (the mother of-peai 1 oys- 
ter) lioes freqviently produce pearl.s, but it differs in 
many impoitant |) uticulai's from the other pearl- 
bearing species of Meleurjiina — the shells of which 
are not, asarnU-, of any value as mother-of-pearl. 
The geographical ilistribulion of the pearl-bear- 
ing species of Ackulidir ajipears to be as 
follows : — 
1. Avicula (Mclea<iriiia) margaritiferu, Lin. — 
The large tropical pearl oyster of Noithern 
Australia, the Pacific, and the Souih Seas 
generally. It is the motiier-of-pearl shell of 
commerce, and although it often contains pearls it 
s chiefly (ished for the sake of its shell alone. 
2 A. (M.) imbikata, Rve. — '1 he pearl shell of 
the northern coast of Western Australia. It is 
said to be closely allied to the Pi.narna species. 
It produces pearls, but its shell is of little 
value. 
3. A. (M.) fimbriafa, Dkr. — The pearl oyster 
of the Gulf of California and ( be Bay of Panama. 
-1. A. (M.) Mart emit. Dkr. — The pearl oyster 
of the Japanese Seas. 
5. A. {M.} fiicafa, Gould. — The pearl oyster of 
the Gulf of Alannar. It also occurs in the Torres 
Straits ,and (probably) in the ArabiinSeas.* 
The pearls produced by the above five species 
are of every shade of colour, from the pale golden 
pearl of Northern Australia to the lustrous black 
gem of the Bay of Panama. Those most generally 
admired however, ami which command the highest 
prices, are of almost transparent whiteness, with 
a slightly azure reflection. They are known as 
the "pearls of the Orient," and are produced by 
the pearl oyster {M. fucata) of the Gulf of 
Mannar. 
NATURE AND FORMATION OF PEARLS. 
It will be necessary for us to pause heie for a 
moment to consider what is up to the present 
time known of the nature and formation of pearls, 
in order that this discussion may have a sound 
basis. 
* Mr. Holdsworth, the Naturalist engaged by the 
Ceylon Government in 1867 to investigate the pearl 
oyster question (but who unfortunately never had an 
opportunity of seeing a fishery, and, according to Sir 
William Twynam, never saw a pear) bank with oysters 
on it), was the fitst to point out the distinction be- 
tweeu the Gulf of Mannar oyster and that of the 
Persian and Arabian Sea,s (Placuna placenta). Some 
shells from the Persian Gulf shown to Captain Donnan 
appeared to him to exhibit no differences from the 
Mannar species ['' Gol(3, Gems, and Pearls." p. 374] — 
from which I conclude, having regard to his (Captain 
Donnan's) great experience, that the Mannar oyster 
also occurs, though perhaps sparingly, in the Arabian 
Seas ; and this may account for the occasional dis« 
fiovery p{ &pe Oriental " pearls ia that region. 
