THE NAUTILUS. 23 
S. securis was described by Temple Prime in the Proc. Bost. Soc. 
Nat. Hist., x, 160, 1851. 
(To be continued.) 
BRIEF NOTES ON THE LAND AND FRESH-WATER SHELLS OF MERCER 
COUNTY, ILL. 
BY WILLIAM A. MARSH. 
97.—Planorbis lentus ? trivolvis var, fallax Haldeman. 
This shell seems to be quite distinct from P. trivolvis, and at pres- 
ent I am unable to decide on its case. That eminent conchologist, 
Dr. James Lewis, identified it as the true Planorbis lentus of Say, 
and until within a few years I have always given it out in exchange 
with that name. I find a great difference of opinion among our best 
conchologists relative to this shell, some considering it trivolvis, 
others Mr. Haldeman’s var. fa//ax, and others regard it as lentus 
Say, while still others regard it P. glabratus. It is our most common 
Planorbis, and is very abundant in the sloughs and lakes of the 
Mississippi River. It is often found associated with trivolvis, but 
may easily be distinguished from that shell by its smoother and 
more shiny surface, finer strie, darker color, in having the labrum 
less prominent above, in having one or more turns in its volutions, 
in being rounded above and below, and in being more depressed. 
98.—Planorbis (Helisoma) bicarinatus Say. 
Shell pale yellow or brownish, subcarinate above and beneath, 
translucent, spire sunken, umbilicate, forming a cavity as deep as 
the base; aperture large and much vaulted above, within reddish- 
brown; whorls three, wrinkled, and with minute revolving lines. 
This species has also a very wide distribution, ranging from British 
America to Kansas. It is found here only in the lakes of the Bay 
Island, in the river and river sloughs. 
99.—Planorbis (Menetus) exacutus Say. 
Shell depressed, light horn color, often almost white, with four 
flattened whorls. Sides obliquely descending to an acute lateral 
edge below the middle; spire not impressed ; aperture transversely 
triangular, labrum angulated in the middle, arcuated near its inferior 
tip, the superior termination just including the acute edge of the 
penultimate whorl. This fine shell is found around the margins of 
