o 
290 THE CHICAGO ACADEMY OF SCIENCES. 
teeth and the side cusps become much larger, so that all three 
cusps are about equal; in addition to this the outer edge of the 
cusp develops several small denticles; marginal teeth long and 
narrow, with three small cusps at the distal end, and several 
(three to four) small denticles on the outer edge ( Fig. 96). 
Genitaha: Not examined. 
Distribution: Eastern North America, west to the Rocky 
Mountains; Manitoba. (Hanham.) 
Geological distribution: Pleistocene; Loess. 
Habitat: Found abundantly in the small streams and 
large bodies of water, either crawling on the muddy bottom or 
on sticks or other submerged objects, including vegetation. 
Remarks: This is the most common, and at the same 
time one of the most variable, species found in the area under 
consideration. It is subject to much distortion, and several 
malformations have been given specific names (p/exata, distorta); 
its size is extremely variable, and giant, or especially corpulent 
specimens, or those with an expanded peristome, have been 
described as distinct (macrostomus). It may always be distin- 
guished by its sharp, simple lip, and by both sides showing al- 
most the same number of whorls. The young shell looks like 
a Physa with the apex cut off. The animal is quite rapid in 
movement, and it is an interesting sight to observe one of these 
creatures moving along the glass side of an aquarium, with 
its long, filiform tentacles waving about, and its shell carried 
perfectly perpendicular. Dr. Sterki (Nautilus, Vol. V, p. 94), 
speaks of the peculiar “stepping” of this species. He says: 
‘He goes a ‘step,’ as far as he conveniently can, with the shell 
deep down and close to the head; then he pushes it forward 
and upward, thus shoving the sand away and making room for 
another ‘step.’ It is more than probable that this digging is 
done not merely for locomotion—for he could do it much 
easier—but in search of food.” The other species also per- 
form this “stepping” act. The species is universally distributed 
throughout the area. During progression the shell is carried 
almost perfectly perpendicular, and the spire and base become 
the right and left side. 
115. Planorbis truncatus Miles, pl. xxx, fig. 28. 
Planorbis truncatus MILES,in WINCHELL’S Geological Survey, Mich., 
p. 238, 1861. 
Shell; Sinistral, orbicularly discoidal, angulated and flat 
above, subcarinated and rounded below; color light chestnut or 
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