76 SMITHSONIAN MISCELLANEOUS COLLECTIONS [voL. 48 
only, is enveloped in a delicate membrane called the mantle. This 
organ, although so simple in structure, is a most important one, 
because, besides other uses to the animal, it forms the shell by 
secreting a milky substance which exudes, mostly from its free 
edges, and hardens layer upon layer, until the shells have reached 
their full thickness and, with the animal, their full size. It is in, or 
upon, the mantle also that true and valuable pearls are sometimes 
formed. Although these soft parts differ more or less in details 
of structure in different genera and species of the Unionide, it is the 
structure and texture of their shells which are generally used in 
recognizing their systematic relationships as well as largely in their 
classification; and it is of course those features and properties of 
the shell alone that are used in the classification of the fossil species. 
In structure the shell of each individual consists of two convex 
valves which are generally equal in size, and of symmetrical shape. 
They are held together at their upper edges by a horny ligament, and 
are also drawn together by the two strong muscles of the animal 
within. Their joined edges under the ligament are usually provided 
with interlocking projections, the so-called hinge teeth, but some 
have no such teeth. The free, or lower, edges of the valves open 
a little way, and it is upon the forward portion of these edges that 
the shell rests when it is in its natural position. 
The shell-substance consists of three distinct layers, each being 
of different quality and texture. First, a more or less thick inner 
pearly layer, which is usually iridescent and often of beautiful tints ; 
second, a very thin vertically prismatic layer outside of, and firmly 
adhering to the pearly one; and, third, outside of all, a thinner horny 
layer, called the epidermis. This shell-structure is the same for all 
the members of the Unionidz in all parts of the world, and it was the 
same for all members of the family that have existed in former 
geological ages, as is shown by their fossil remains. 
Although the family characteristics of the Unionide, in whatever 
part of the world they are found, are clearly defined by structure and 
shell-texture, the species and genera in certain great regions are dis- 
tinctly different from those of other regions. The family is of world- 
wide distribution, representatives of it being found in the fresh waters 
of all the continents, in those of all the large islands, and in those 
of some of the smaller sea-girt islands. The number of known 
species of the family now living in the whole world is about one 
thousand. Of this number about six hundred species live in North 
American waters, and of the latter number the Mississippi River 
system alone contains about four hundred, or about four tenths of all 
the known species in the world. 
