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 specimei 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, a small 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 bookless type and develop in the mouth of fish 

 and the undulatory motions possibly attract these fish which may be thus 



