does not extend beyond 400
north latitude. This species is likewise small, and closely resembles
the pearl-oyster of Ceylon, from which it differs principally in
coloration. As noted by Jameson, brown and white predominate in the
exterior coloring, and the interior of the lip is marbled with yellow
ocher and chocolate brown, instead of pink, as in the Ceylon shell.
There
are numerous other species of pearl-oysters, but they are of slight
economic importance, and do not support fisheries of value.
As
only a small percentage of the individual mollusks contain pearls, it
follows that vast quantities are destroyed without any return
whatever, and handling them merely adds to the expense of the industry,
as well as reduces the resources of the reefs. This could be obviated
if it were possible, without opening them, to determine the individual
mollusks containing pearls.
Among
the several methods proposed for this purpose, especially interesting
is the use of X-rays, which was suggested by Raphael Dubois of Lyons,
France, in 1901.1 The shells of some pearl-oysters— those of
Ceylon and of Venezuela for instance—are relatively thin, and it was
thought that by the means of the rays the presence of pearls could be
ascertained, and non-pearl-bearers could be saved from opening, and be
returned to the reefs without injury. Although the calcareous shell
partly interrupts the radiations, it is not difficult to recognize the
presence of large pearls.
The
theory has never been found practical in application, owing largely to
the rough and irregular exterior of the shell and the small size of the
pearls. The presence of the larger pearls may be ascertained by this
method ; but it is exceedingly probable that a very large percentage of
the small ones, and especially the seed-pearls, would be overlooked.
Furthermore, if in their sixth year oysters contain no pearls, the
probability of appearance therein later is very small, and little
benefit would result from their return to the water. As to saving the
trouble of opening the non-pearl-bearing mollusks, labor in the
pearling regions is usually inexpensive, and this cost is far more than
offset by the reasonable certainty of securing practically all the
small as well as the large pearls by the present method of operation.
Owing to the greater thickness and the economic value of the large
pearl-oysters—as those of Australia or of Mexico, for instance—the
application of X-rays to them is obviously impractical. However, when
pearl-oyster culture becomes a highly developed industry, with
personal ownership in those mollusks returned to the water, some
method such as this might be of great value.
Pearls are yielded by various species of Unionidee or Naiades occur-
iSee"ComptesRendusderAcadémiedesSciences,"Paris, 1904, Vol.CXXXVIII.pp. 301,302.
