20 BULLETIN OF THE BUREAU OF FISHERIES. 
the gill pouches of the female, passed out to become parasitic upon fish, and liberated 
after the period of parasitism, all within a relatively short period of weeks or months 
and generally during the summer season. Most of these species are tolerably restricted 
in their parasitism, and for this reason, as well as on account of the short breeding 
season, they lend themselves less readily to propagation by artificial means. The chief 
dependence for their conservation must now be placed upon protective measures in 
order to insure a plentiful supply of spawners in nature, and, as is equally important, 
upon efforts to promote the abundance of the fishes upon which the mussels become 
parasitic. These mussels were not the first to be used and appreciated, but after coming 
into use their popularity grew until in recent years they have constituted the greatest 
portion of the raw material for manufacture. Other species of mussels fell into dis- 
favor, but now, with the discovery of better material in the Lampsilis class, the pioneer 
mussels in commerce are again returning to favor. 
From the best to the worst there is a wide extreme, but, generally speaking, Quadrula 
shells are harder and of better luster and iridescence than others; these superior qualities 
are doubtless associated with their comparatively slow rate of growth. The individual 
shells show greater extremes of thickness than Lampsilis mussels (such as the mucket) 
so that, in cutting and finishing buttons from them, there is a relatively high proportion 
of waste. In addition to the relative ease of propagation, therefore, there are several 
practical advantages in favor of the Lampsilis mussels. 
NIGGERHEAD GROUP. 
The shells of the niggerhead group distinguish themselves from all others of the 
Quadrula class by combining a smooth exterior surface with a high degree of uniformity 
of quality. The niggerhead takes first place among the Quadrulas. 
The niggerhead, Quadrula ebenus (Lea) (Pls. II and III) came to be the mussel 
most sought, and a few years ago, at least, it was the commion standard of value. The 
better shells were suited to the export demand and accordingly have advanced in price. 
Its preeminent qualities, as compared with other species of this and the following 
groups, are its clear, pearly luster (equaled by only a few), the relative thickness of the 
iridescent portion, and its abundance in favorable streams. The nacre is of fine regular 
grain and lustrous white, except where iridescent. In buying mussels for button manu- 
facture the price is often based upon the percentage of niggerheads. 
The niggerhead forms 80 per cent of some mussel beds of the Mississippi but is not 
so common in the tributaries. It is usually restricted to the larger streams. It was 
once thought that the Mississippi niggerheads were better than any other, but this is not 
always the case. Some of the niggerheads from Arkansas are unsurpassed, especially 
those of the St. Francis River. Like other mussels, the niggerhead varies quite a little 
in form. Those which are more flat and round are preferred (PI. III, middle shell), as 
compared with examples which are elongate (Pl. III, upper shell) or in which there is 
a noticeable step-off from the thicker forward and central portions to the thinner hinder 
third of the shell. The shells formerly taken in the Des Moines Rapids above Keokuk 
were of the better character. 
A notable feature of the niggerhead mussel is the markedly iridescent hinder portion 
of the shell, and the thickness of this portion is generally better sustained in the nigger- 
head mussel than in any other species displaying iridescence. The buttons finished from 
