124 THE NAUTILUS. 
Height, 5; greatest diameter, 5; aperture height, 5; breadth, 
2.6mm. Young, 3 whorls. 
(Collection Mus. Nat. Hist., U. of I., No. Z11393A.) 
This Planorbis differs from typical trivolvis in being less high 
in comparison with its diameter, in the separation of the last 
whorl, above, from the carina of the preceding whorl leaving a 
V-shaped trough, which is not present in trivolvis, and in show- 
ing three full whorls on the umbilical side while in trivolvis 
there are but two full whorls. The sculpture is also more 
regular than in trivolvis, the rib-strie being more clear cut with 
wider interstices. The carina on the upper whorls in pseudo- 
trivolvis is also sharper and forms a raised keel bordering the 
spire whorls. 
This Planorbis has perplexed Illinois conchologists for many 
years, being uncertainly referred to Say’s glabratus as figured by 
Haldeman in the Monograph, plate 2. Whether all of the 
shells listed under this name in the Illinois Catalogue (p. 106) 
are referable to the new form is not known, specimens from 
these localities not being available for examination. The same 
Planorbis occurs in Pleistocene deposits in and about Chicago 
and has been referred to trivolvis in papers and references (cf. 
Trans. Ill. State Acad. Sci., iv, p. 112). The fossil specimens 
referred to this species occur at the following places (see the 
writer’s Life of the Pleistocene, now in press by the University 
of Illinois, for the data concerning these and other sedimentary 
strata in the Chicago region): ; 
200 feet north Dempster Street, station 47, stratum ix, silt. 
200 feet south Dempster Street, station 45, stratum iv, silt. 
200 feet north Oakton Avenue, station 42, stratum vii, silt. 
Lemont, Lincoln Park extension office, Santa Fé R. R., 
stratum il, silt. 
Two fossil specimens measure as follows: 
Height, 8; greatest diameter, 23; aperture height, 8; breadth, 
8mm. No. P396 (Chicago). 
Height, 9; greatest diameter, 21; aperture height, 9; breadth, 
9mm. No. P401 (Lemont). 
Pseudotrivolvis is not found in the earlier deposits in Wilmette 
Bay, Chicago, the Planorbis there being true trivolvis, while in 
