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). 



Genitalia: 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 (plexata,distorta); 

 its size is extremely variable, and giant, or especially corpulent 

 specimens, or those with an expanded peristome, have been 

 described as distinct (macro stomus} . 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, pi. 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 



