FRESH-WATER MUSSELS AND MUSSEL INDUSTRIES. 23 
deep purple of that portion of the nacre within the pallial line makes it valueless for 
buttons.” 
PIMPLE-BACK GROUP. 
The better shells of this group have the same general qualities as the niggerhead. 
‘The best are inferior to the niggerhead only in that the backs are rough or warty, and the 
thickness of the tip is less well sustained. The luster is fine, and a portion of the shell 
is iridescent. The poorer shells of the group are worthless because of color of nacre, 
shape, or some other objectionable quality. 
Like the niggerhead, they are short-time (summer) breeders and tolerably restricted 
in parasitism, as far as is known. Unlike the niggerhead, they rarely, if ever, occur in 
such numbers as to constitute the principal species in a mussel bed. 
The pimple-back, Quadrula pustulosa (Lea) (Pl. V), is distributed throughout the 
whole Mississippi Basin in different forms and sizes but does not often attain a length 
greater than 2.5 inches. It occurs mixed in with other species and sometimes forms 
10 per cent of the mussels in the beds. It is one of the best mussels of the Illinois River. 
It varies greatly as regards the size and number of pustules and rarely displays an almost 
entirely smooth shell. Its diversity of form is well illustrated by the several figures in 
Plate V. 
In the earlier years of the industry the pimple-back was not used. The workmen 
did not like it on account of the pustules on the back, which made it difficult to cut. 
Later, as button-making material became scarcer, it came into use and is now bought 
and worked up along with the niggerhead, having the same market value. 
The texture is firm, and the shell has a tolerably uniform thickness; since its thick- 
ness diminishes rather uniformly toward the tip, it can be worked up economically. 
It is principally used for small-sized buttons. The color of the nacre is lustrous white, 
and there is a fine iridescence in the hinder portion. 
The pimple-back spawns in early summer and midsummer, and the glochidia are 
parasitic, chiefly upon several species of catfish. 
Quadrula pustulata (Lea) (Pl. V1) is like pustulosa but is smaller and with fewer 
warts. It is comparatively rare and is not distinguished commercially from pustulosa. 
Quadrula cooperiana (1,ea) (Pl. V1) is a more southern form of pimple-back found in 
the Cumberland and Tennessee systems. It is called pimple-back, but, unlike the north- 
ern form, the nacre may be white or from a pale to a deep shade of pink. A blank of 
from 30 to 36 lines can be cut from the white shells. 
The maple-leaf, Quadrula lachrymosa (Lea) (Pl. VII), is not found in great quan- 
tities but occurs in small numbers among other mussels; for this reason it was once 
known as the “stranger.” The material is of a good, white luster and firm texture, 
but, owing to the thin tips, about half of the blanks can be used only as ‘‘tips,” which 
is the commercial term for blanks less than one-twentieth of an inch in thickness.? 
A small proportion of iridescents is obtained, and, but for the thinnish tips and 
knobby back, the shell would be equal to that of the niggerhead. When found in 
considerable numbers in the shell piles at the cutting plants they are sometimes sorted 
out and cut separately. 
@ See footnote, p. 17. 
