124 THE NAUTILUS. 



Height, 5; greatest diameter, 5; aperture height, 5; breadth, 

 2.5 mrn. 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-striae 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. 111. 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 Fe R. R. , 

 stratum ii, silt. 



Two fossil specimens measure as follows: 



Height, 8; greatest diameter, 23; aperture height, 8; breadth, 



8 mm. No. P396 (Chicago). 



Height, 9; greatest diameter, 21; aperture height, 9; breadth, 



9 mm. No. P401 (Lemont). 



Pseudotrivolvis is not found in the earlier deposits in Wilmette 

 Bay, Chicago, the Planorbis there being true trivolvis, while in 



