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SOURCES OF PEARLS 67
—at
least in the first years of growth—also distinguish them from the
sedentary edible oysters. But from an economic point of view, the
principal difference is the possession of a thick, nacreous, interior
lining in the shells of pearl-oysters, which is wholly lacking in the
edible species. Like their namesakes, the pearl-oysters are
exceedingly fertile, a single specimen numbering its annual increase
by millions.
Commercially
considered, the pearl-oysters are roughly divisible into two groups,
(1) those fished exclusively for the pearls which they contain, and (2)
those whose shells are so thick as to give them sufficient value to
warrant their capture independently of the yield of pearls. The best
examples of the first group are the pearl-oysters of Ceylon and of
Venezuela, and to a less extent those of the Persian Gulf, the coast of
Japan, and of Sharks Bay, on the Australian coast. Of the second group,
the pearl-oysters of Torres Straits and of the Malay Archipelago are
the most prominent members. Between these two groups are the many
species and varieties whose shells and pearls are more evenly divided
with respect to value, including those of Mexico, Panama, the Red Sea,
the South Sea islands, etc.
Some conchologists recognize a large number of species of Margari-tiferœ, while
other authorities consider many of these as local variations of the
same species. There is much difference in the size, color, and markings
of the shells in different localities, owing to varying geographical
and physical conditions. The distinction of species and the
nomenclature herein adopted are those of Dr. H. L. Jameson, who has
recently revised and rearranged the collection of shells belonging to
this family in the British Museum of Natural History,1 and to whom we are indebted for descriptive notes relative to several of the species.
The greatest pearl-producer in the family of pearl-oysters is the Margaritifera vulgaris of
the Gulf of Manaar and the Persian Gulf, and to a much less extent of
the Red Sea. It occurs in various other inshore waters of the Indian
Ocean, and about the Malay Archipelago and the coast of Australia and
New Guinea, although it is not the principal pearl-oyster of those
waters. An interesting account of its immigration into the
Mediterranean Sea through the Suez Canal was given by Vassel in 1896.2
This
species is quite small, averaging two and a half inches in diameter in
Ceylon waters, and somewhat more in the Persian Gulf,
1 Jameson, "On the identity and distribu- 2 Vassel, "Sur la Pintadine du Golfe de
tion of the mother-of-pearl oysters; with a Gabes, Comptes Rendus Assoc. Franc.," revision of the subgenus Margaritifera." 1896, pp. 458-466. Proceedings of the Zoological Society of London, Vol. L, 1901, pp. 372-394.
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