378 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
has a wide field for imagination. The pearls, however, have but slight value unless 
they are beautiful and lustrous. 
Frequently pearls have an opaque appearance and seem to be worthless, but on the 
removal of their outer layer are found to be clear and iridescent. This outer layer 
may be removed by dipping them in a weak solution of acid, which dissolves the 
opaque coating, or it maybe peeled with a knife, although sometimes the pearl is not of 
the same material throughout and can not be restored. The story is told of a New 
York lady who purchased a button-shaped Unio pearl that had a black, diseased 
appearance on one side. It was so set that the imperfection was all below the mount- 
ing. When applauding at the opera one evening the pearl was broken, and on 
examination it was found to consist of a very thin nacreous layer, inside of which was 
nothing but a hard, white, greasy clay. (See plate x; enlarged 3 diameters.) 
Whatever be the method of their formation, it would seem that pearls are formed 
at the expense of the shell, for the substance necessary to their growth is drawn from 
sources which normally secrete the shell. Hence the presence of a pearl can some- 
times be detected on the outside of the shell. Normal appearing shells rarely contain 
pearls, while on the other hand those that are deformed often contain pearls of great 
beauty. There are three indications on which pearl-fishers to some extent rely for 
detecting the presence of pearls from the outward aspect of the shell. These are, 
first, the thread — that is, a recess or elevation extending from the vertex to the edge; 
second, the kidney shape of the shell — that is, an indentation on the ventral side; and 
third, the contortion of both valves toward the middle plane of the animal. 
The precise manner in which pearls are formed is a matter of some uncertainty, 
and several views are held, all of which have some apparent basis in observed facts. 
There are three principal theories, viz, that the special and unusual secretion of the 
pearly material at certain points is due, first, to disease; second, to accidental injury, 
and third, to the intrusion of foreign substances of some kind into the shell. The 
first view is sustained by the fact that pearl production seems to occur in certain 
streams and at certain periods especially, as though it were a result of some peculiar 
condition affecting the shells largely at certain times, like an epidemic disease; and 
it has also a slight analogy in the development of calculi and of gout in higher 
animals. The second theory, that of injury or accident, is largely based upon the 
frequent occurrence of pearls in shells that have an aspect of distortion or deformity. 
This, however, is very far from being universal, and might also be a result of disease 
rather than of accident. The third view, that pearls are caused by the intrusion of 
foreign bodies, which the mollusk, if unable to expel, covers over and incloses with 
the pearly secretion, has the evidence of actual demonstration in many instances and 
is unquestionably true to a large extent. It maybe, however, that the other theories, 
particularly that of disease, are also true in some degree, and that pearls may be 
formed in either of several ways. 
Still another view is held by some, which lies rather between the first and third of 
those already mentioned, viz, that the nucleus of a pearl is an egg of the mollusk, 
which has for some reason failed to be expelled in the usual manner. The ova in the 
Unios are kept for some time in the outer pair of gills prior to being discharged 
into the water, and it is quite possible that some of them may occasionally be caught 
in the gill tubes and not be able to escape. In such a case the entangled egg may be 
coated over with nacreous material and form a “free” pearl. This, of course, would at r 
first be very small and its growth would be due to a continued irritation, producing 
