22 BULLETIN OF THE BUREAU OF FISHERIES. 
Because of the demand for the smaller sizes, a large number of very small shells are 
being marketed, many of them entirely too small to be of any service whatever. 
In the years just preceding 1914 there was a growing export trade in niggerhead 
shells of small and medium size, the price reaching $40 per ton on the river. In conse- 
quence of the export demand, the domestic market was diverted more and more to the 
inferior grade of shells. Since 1914 the domestic market has found a larger supply of the 
niggerheads available to it, and consequently the domestic demand for lower-grade 
shells has declined. During several years prior to 1914 the prices paid for niggerhead 
shells for domestic manufacture varied from $18 to $27 per ton; in 1919 the price per 
ton ranges from $40 to $80. 
The niggerhead mussel appears to have two spawning periods, one in spring and 
another in early and midsummer, but the periods are yet to be accurately defined if they 
are actually distinct. Like other mussels, the niggerhead is parasitic upon fish, but the 
only species of fish known to carry it successfully is the river herring, Pomolobus chry- 
sochloris. Since this fish is characteristic of deeper and swifter streams, the distribution of 
the niggerhead mussel is restricted accordingly. Evenin sucha large but generally sluggish 
river as the Illinois the niggerhead is rare, and Forbes and Richardson report that the 
river herring is very uncommon in that stream. There are many herring in Lake Pepin, 
but few niggerheads are taken there, so that other conditions must be unfavorable in 
this place. The niggerhead is generally found in hard, gravelly or rocky bottoms, and 
it is very abundant in such rapids as occur on the Mississippi. Its distribution is, how- 
ever, rather hard to define, since some of the larger examples have been taken in deep 
and slowly flowing water. The White River of Indiana, the Scioto River of Ohio, and 
the Duck River of Tennessee have yielded some particularly large shells. 
Quadrula solida (Lea) (Pl. 11) is very like Quadrula ebenus and is generally regarded 
by mussel fishermen as the same. The material is equal to that of the niggerhead. 
Although widely distributed through the Mississippi Basin, the mussel is relatively 
rare and small and can not be rated as of much importance. 
Quadrula subrotunda (1,ea) (Pl. IV) is found in the Ohio, Cumberland, and Tennessee 
River systems. It resembles the niggerhead, and the adults are difficult to distinguish 
from the latter. At Clarksville, Tenn., it is called the ‘“‘long solid,”’ and is regarded as 
one of the best button shells of the lower Cumberland. 
The hickory-nut, Obovaria ellipsis (Lea) (Pl. III) must be grouped with the nigger- 
head in respect to commercial qualities, although it is not closely related to it. This 
is perhaps the only conspicuous case in which different mussels agree closely in 
quality of shell while being rather distantly related in systematic characters. The 
hickory-nut mussel possesses a shell of essentially the same quality as the nigger- 
head and has sometimes been called the Missouri niggerhead. ‘The mussel differs 
from the Quadrulas as a class in being a long-term breeder, carrying the young in the 
marsupial pouches over the winter period. 
Closely related species are Obovaria circulus (Lea) and Obovaria retusa (Lamarck). 
The former is found principally in the southwestern portion of the basin. It is too small 
to be of much commercial significance, but will yield a few small blanks. The golf-stick, 
Obovaria retusa (Lamarck), is found in the Ohio, Cumberland, and Tennessee systems. 
Wilson and Clark report that it attains a rather large size in the Cumberland, ‘“‘but the 
