By! THE NAUTILUS. 
BRIEF NOTES ON THE LAND AND FRESH-WATER SHELLS OF MERCER 
COUNTY, ILL. 
BY WILLIAM A. MARSH. 
Sub-genus Planorbula, Haldeman. 
101.—Segmentina armigera, Say. 
Shell varied in color, from very dark horn color to almost white, 
spire regular, slightly concave, suture well impressed, whorls four, 
longer than wide, carinated above, aperture oblique, labrum 
usually of a darker color on the edge. Within the aperture are five 
teeth, two on the pillar lip, one near the anterior lip, one on the side 
of the labrum, and two on the upper portion of the outer lip. This 
interesting shell has a range from Maine to Colorado. It is rather 
common here about our small ponds, being usually found associated 
with P. exacutus and P.parvus. It may be found adhering to sticks, 
bark and pieces of pine boards where the water is very shallow. 
ANCYLUS Geoffrey, 1767. 
102.—Aneylus rivularis, Say. 
Shell pale yellow, opaque conic, depressed ; apex obtuse, nearer to 
and leaning towards one side and one end; aperture oval, rather 
narrower at one end, entire. This shell is very abundant along the 
margin of the Mississippi River; found adhering to the limestone 
rock in the river, also on limbs of trees, dead leaves and old valves 
of Uniones. I have one valve of a Unio ellipsis in my cabinet that 
had 25 of the Ancylus on it. 
103. 
Ancylus parallelus, Haldeman. 
Shell pale, thin, fragile, lengthened, narrow, apex rather short, 
sharp, conspicuous, with two-fifths of the shell posterior to it. Inhabits 
Pope Creek, in this county; has never been found elsewhere. 
When found at all it occurs very abundantly, adhering to sticks, 
stones and dried leaves. I have found it attached to live specimens 
of Pleurocera subulare. 
104.—Ancylus tardus, Say. 
Shell conic, depressed, apex behind the middle, obtuse, rounded, 
inclining backward, line from the apex to the posterior tip rectilin- 
ear; line from the apex to the anterior tip arcuated ; aperture oval. 
