36 BULLETIN OF THE BUREAU OF FISHERIES. 
In nearly every case the little mollusks thus found are not the young of larger mussels, 
nor are they mussels of any character. It should be clearly and generally understood 
that, while all the larger clams of the rivers are mussels, there are clams which are not 
mussels at all, being markedly different in structure and development. There is no 
common name for these, but they pertain to several species of Spherium, Musculium, 
and Corneocyclas (Pisidium) (Pl. XXVI). The usual diameter is 0.25 to 0.5 inch. They 
are frequently found attached to stones or to old shells in the mud. A dead shell may 
be found to be full of them. The Unionide (fresh-water mussels) and the Spheeriide 
live side by side in our rivers, but each family is more nearly related to different families 
of ocean shells than to each other. 
SUMMARY. 
Of the North American species of fresh-water mussels, more than 500 in all, we 
have named 66 as more or less familiar to the fishermen, but of these only 41 can be 
classed as having commercial value in the shell trade. Some of the others are valuable 
as producers of pearls. 
Looking at the 41 used in manufacture more closely we find only 17 that are of 
real importance at the present time. It is desirable to name these separately. 
Quadrula class: Lampsilis class: 
Niggerhead. Mucket. 
Hickory-nut. Southern mucket. 
Pimple-back. Lake Pepin mucket (fat mucket). 
Maple-leaf. Butterfly. 
Monkey-face. Pocketbook. 
Pig-toe. Yellow sand-shell. 
Ohio River pig-toe. Black sand-shell. 
Blue-point. 
Three-ridge. 
Washboard. 
Many manufacturers or buyers would reduce this list by omitting several of the 
species, but there would probably be little agreement as to the species to be eliminated. 
The best of all species at the present time are the yellow sand-shell, the nigger- 
head, the southern mucket, and the Lake Pepin mucket, the last three being of approx- 
imately equal value. The yellow sand-shell has been used entirely for export and 
commands a price nearly double that of other species. Many niggerheads were 
exported a few years ago, causing a distinctly advanced price. 
There is a great deal of variation in quality among the several species. Some 
shells are better for one purpose, while others are better for another. 
Within the species there is variation according to the locality in which the mussels 
have grown. A mussel may have a shell of good quality in one stream and of poor 
quality in another. The differences may apply to color, luster, texture (firm, chalky, 
brittle, or hard), and form (shape and thickness). 
Within the same mussel bed there may be differences in quality in the same species. 
We may find side by side pink muckets, white muckets, etc. 
The shells of the Quadrula class show more uniformity in quality over the entire 
region of distribution than those of the Lampsilis class. 
Iridescence is best shown in the niggerhead and pimpleback groups, but only a very 
small percentage of truly iridescent buttons can be obtained in any case. 
