376 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
and Cretaceous deposits. Among the gasteropods the pearly groups are the turbos 
and haliotes or abalones, in both of which, but especially in the latter, there is a fre- 
quent occurrence of green iridescence. Shells of both these families are u cleaned” with 
acids for use as ornaments, and the exquisite green Haliotis material is extensively 
used in the arts under the name of abalone. 
The pearls of commerce, however, are almost wholly obtained from bivalve (lamel 
librancli) shells, of which the following families have a nacreous lining: Aviculidce , 
Mytilidce , and Unionidce , the last being a l'resh-water group, also known as the 
Naiades. A few genera of other families are also brilliantly pearly, but need not be 
here discussed. The true pearl oyster ( Meleagrina ) found in the Pacific and Indian 
oceans belongs to the first of these families, and has from time immemorial yielded 
the bulk of commercial pearls, while its large and thick shell furnishes the mother-of- 
pearl for countless ornamental purposes. The Naiades are of particular interest in 
this country, as it is in North America that this group is most abundant. Several 
hundred species of Unio, Anodon, etc., have been found in our great rivers and lakes, 
and the Mississippi basin teems with them, in forms, for the most part, quite distinct 
from those of the Atlantic watershed and of the Old World. The Unios, while all 
iridescent, vary greatly in tint, exhibiting many delicate shades of pink, brown, 
purple, etc., as well as white. The rivers of Europe, of Mesopotamia, and of China 
also yield large numbers of Unios, while other allied genera ( Syria and Casialia) 
represent the family in the Amazon basin of South America. 
In the fresh-water species the two valves are alike in size and shape, while in 
some of the marine families they differ, as is well seen in the common oyster. Each 
of the valves consists of two parts, the epidermis and the shell proper, the latter 
composed of numerous layers. The epidermis, which resembles horn, consists 
chiefly of a brown or yellow substance called “ conchioline/’ soluble in caustic alkalies ; 
beneath this is the outer portion of the shell proper, the prism stratum, consisting 
of layers formed of minute prisms arranged vertically to the layers and the shell 
surface; and, third, the interior nacre layer, composed of finely folded leaves parallel 
to the shell surface. The last two strata consist chiefly of carbonate of lime. These 
formations may be seen in transverse cuttings and microscopic sections. The soft 
internal parts of these mollusks are covered by a thin, delicate membrane called the 
mantle, from the surface and particularly from the outer edges of which material is 
excreted to form the inner layers of the shell. Whenever, by accidental injury, 
disease, or intrusion of foreign substances, local irritation is set up in these tissues, 
the effect is to produce an increased secretion of the nacreous matter at this point, 
resulting in the formation of pearls or pearly concretions. 
Pearls are of several distinct kinds, differing in shape and perhaps, as elsewhere 
suggested, in origin. These are (first) what are known as “free” pearls — those that 
are found loose and separate between the folds or layers of the mantle and gills, or 
between the latter and the body of the mollusk. These comprise most of the true 
spherical pearls, as also many that are ovate, pear-shaped, and irregular. Then there 
are the pearls found between the mantle and the valves of the shell ; these, if free 
at all, are apt to be hemispherical, or in any case flattened on the side toward the 
shell, while very often they are attached more or less to the valve by a deposit of the 
pearly secretion. In the region of the hinge these become extremely irregular in 
shape and often greatly elongated, forming a third kind, known as hinge pearls, 
baroques, etc. 
