PEARLS AND PEARL FISHERIES. 
379 
an abnormal secretion of the pearl material in the adjacent tissues. It is evident 
also that pearls of this kind could be formed only in the female shells, and this point 
is one that requires further investigation. 
The evidence for the intrusion theory may be briefly reviewed as follows : Cases 
are known among the marine pearl oysters in which small fish entering the open 
valves of the large shell have worked their way in between the shell and the mantle 
and been unable to escape. They have then been coated over with the pearl secretion 
and fastened down thereby to the inner surface of the valve. When subsequently the 
shell has been gathered and opened by pearl-fishers the form of the little intruder 
has been found distinctly preserved in pearly relief on the interior of the shell. Other 
similar instances are also known. 
Among many remarkable specimens of pearls and pearl shells exhibited at the 
World’s Fair at Chicago in 1893, and now in the Field Columbian Museum, were 
several examples of this kind. One of these was a small piece of true mother-of-pearl 
shell two-fifths of an inch in length, which broke while undergoing the operation of 
being made into a button, revealing a small inclosed crab immediately below the 
blister. Among fresh water shells the same fact has been indicated in a few instances — 
one where a crayfish has been thus inclosed beneath a pearly covering, and another 
where a Unio, from Long Island, contained an insect entombed in the same way. 
There are, however, even more positive proofs. It has long been the habit of the 
Chinese to produce artificial pearl objects by introducing little flat metallic figures, 
usually images of Buddha, between the valves and the mantle of a large river-mussel 
of that country ( Dipsas plicatus). These little figures, made of tin, are carefully 
inserted so as not to injure the animal, which is then returned to the water and left 
for some months or a year. When again dredged up and opened, the figures are found 
to be entirely coated over with the pearly material and slightly attached thereby 
to the inner surface of the valve; they may then be easily removed and used for 
ornaments or charms (plate hi). The Chinese also sometimes insert strings of small 
beads, which become apparently pearls, and carry out this same method by other 
ingenious devices. 
In a shell in the Lea collection of Unionidce , which has been presented to the 
United States National Museum, an oval piece of white wax, flat on the lower side 
and rounded on the upper, which had been inserted in the valve near the hinge, is 
entirely coated with a beautiful pink nacre. It has been broken out of the shell, the 
pearly nacre of the lower or flat side remaining in the shell, whereas the dome-shaped 
piece retains the coating. 
At the International Fisheries Exhibition, held in Berlin during 1880, there were 
shown the results of experiments undertaken in Germany toward the production of 
artificial pearls from Unios, in a manner similar to that practiced by the Chinese. 
Flat tin figures, usually of fish, were introduced between the mantle and the shell. 
Similar experiments were conducted in the Royal Saxon pearl fisheries. Either small 
foreign bodies were introduced into the mantle, in order to form the nucleus for the 
free pearl formation, or the Chinese method of inserting such bodies between the 
mantle and the shell was followed. From the second method successful results were 
shown. The foreign bodies that had been introduced — poor pearls from other mussels, 
pieces of grain, or china buttons — were entirely covered with nacreous substance. 
The shape of these objects makes it impossible for the mantle to fit closely around 
