20 BUlvLETiN OF The bureau O^ 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 efiforts 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. 



NIGGKRHAKD 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 ebentis (Lea) (Pis. II and III) came to be the mussel 

 most sought, and a few years ago, at least, it was the common 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. Ill, middle shell), as 

 compared with examples which are elongate (PI. Ill, 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 ^he nigger- 

 head mussel than in any other species displaying iridescence. The buttons finished from 



