153] FAUNA OF BIG VERMILION RIVER—BAKER 55 
space between the margin and the pallial line. This was evidently formed 
by distomid parasites (Z11058 A). Another specime1 from the same 
lot (Z11058 B) has a large group of pearls (over 100) filling the space 
between the ventral margin and the pallial line over an area of about two 
inches. Other specimens have an abnormally thickened ventral margin 
in addition to blisters, indicating the presence of some irritating material 
between the shell and the mantle lobes, probably mud or sand (Z11149 
A). A specimen from Mahomet had suffered a curious injury which had 
caused a large blister extending from the upper third of the posterior end 
to the center of the ventral margin, a distance of about three inches. This 
was plainly due to the presence of a quantity of mud getting in between 
the shell and the mantle, asmall amount of this material being retained 
under the cylindrical blister on the ventral margin. The outside of the 
shell was so perfectly repaired that there was no evidence of the pathologi- 
cal condition within the shell (Z11202 A). Fig. 35. This injury was only 
in the left valve, the right valve being normal. 
Nearly all of these pearly growths, blisters, and abnormalities have 
been noted to occur almost universally without the pallial line, between 
this muscle attachment and the ventral, anterior, and posterior margins 
of the shell. This limited area is easily understood when it is remembered 
that the adductor muscles and the pallial line form a barrier to the entrance 
of foreign material because the muscles at these points are firmly attached 
to the shell and parasites or foreign material cannot readily gain entrance 
to the interior of the animal between the mantle and the shell. This is 
true of all the shells of other species examined from these two river systems. 
It was noted that pearly growths and abnormalities were rarer in the 
shells from the Middle Fork and from Salt Fork near the Middle Fork 
than from Homer Park and up the stream from this habitat. The shells 
from the Sangamon River at Mahomet were much less subject to parasitism 
or abnormalities than those from the Big Vermilion River. 
Young shells of ventricosa were common only in Middle Fork, in Salt 
Fork near Middle Fork, and in the Sangamon River at Mahomet., No 
young shells were collected from the stream above the dam at Homer Park. 
Gravid females were observed on September 26 in the Salt Fork near 
Middle Fork in the act of spawning. The shells were buried in the sand, 
only the tips of the siphons showing above the general level of the bottom 
of the stream. The bright yellow siphon fringes, which are enlarged during 
the spawning period, were observed to wave about with graceful undula- 
tory motion. The movements were more or less intermittent, a waving 
period being followed by a resting stage. These mussels had well formed 
glochidia and were evidently discharging the embryos from the marsupia. 
These glochidia are of the hookless type and develop in the mouth of fish 
and the undulatory motions possibly attract these fish which may be thus 
( 
