REPRODUCTIOX IN THE UNIONID^ 
83 
Quadrula ebena, illustrates the typical appearance of the marsu- 
pium in this group, although the gills, as they appear in the figure, 
are not as fully distended as is frequently the case. In many 
species of the genus the eggs and embryos are brilliantly colored, 
red or pink, and, when the marsupium is charged, the color shows 
through the transparent walls of the gills, which present a striking 
appearance on removing the shell. 
There has been a certain amount of discussion among the conch- 
ologists as to whether, or not, the functioning of all four gills as 
a marsupium is a constant character in Quadrula, and observa- 
tions have been to a certain extent conflicting. Since Simpson has 
made use of this feature in characterizing the group Tetragenae, 
some importance has been attached to the apparent discrepancy in 
observations. Ortman (/. c, p. 101), for example, states that in a 
dozen specimens of Q. coccinea only the outer gills were charged 
with embryos. ''This would remove" he says ''this species 
from the genus Quadrula, and would place it with Pleurobema. 
Baker ('98, p. 80), gives a description of the soft parts, and says, 
"four gills used as marsupium," but this may not be founded upon 
personal observations, but may have been inferred from the sys- 
tematic position of the species." 
While examining mussels on the upper Mississippi River in the 
summer of 1908, we observed a peculiarity in behavior in all of the 
species of Quadrula collected which probably accounts for the 
conflicting descriptions of the marsupium in this genus, and also 
for the fact that in some species gravid females have never been 
observed at all. Every species of Quadrula that came into our 
hands exhibited to a greater or less degree the habit of aborting 
embryos and glochidia, when taken out of the river, and if they 
were not opened and examined at once upon capture, they were 
general^ found shorth^ afterwards to be either partially or entirely 
empty. Some individuals discharged the contents of their gills 
more readily and completely than others, the abortion involving 
either all four gills, or onl}^ the inner or outer ones, or, again, only 
a portion merely of one or more gills. In the pre-glochidial stages, 
when the embryos are conglutinated, the entire masses were dis- 
charged, while individuals were frequently seen in the act of abort- 
