96 



THE NAUTILUS. 



Height at aperture, 1.75; greatest diameter, 4.25 mm. 

 Cotype. 



Height at aperture, 2.00 ; greatest diameter, 4.00 mm. 

 Cotype. 



Height at aperture, .90; greatest diameter, 2.00 mm. 

 (young, 3 whorls). 



Holotype; number Z 10775 and eotypes number Z 10776, 

 Museum of Natural History, University of Illinois. 



This small Planorbis is related to deflectus, but differs mark- 

 edly in the form of the upper whorls which are more sharply 

 carinated, and in the spire which is more sunken below the 

 general level of the whorls. The umbilical region is deeper 

 and the aperture is higher than wide. The lower part of the 

 body whorl is more exposed below the first half of this whorl 

 than in deflectus. Young specimens very strongly resemble 

 Planorbis camp anulat its in form. 



Specimens of deflectus from marl deposits in Milwaukee 

 (30th Street) Wisconsin, have occasional individuals that 

 somewhat resemble alUssimus in the greatly deflected last 

 whorl but these are otherwise quite different. The new 

 species may be looked for in marl deposits associated with 

 Galba obrussa decampi and the Pisidia peculiar to the north- 

 ern marl beds. Only 5 adult and 9 immature specimens oc- 

 curred in the Urbana marl deposit and the new species was 

 not, seemingly, a common inhabitant of the pond or lake. 



The new forms described above occurred in a lot of post- 

 glacial fossils found in a deposit on the campus of the Uni- 

 versity of Illinois, in a ditch and in excavations for the base- 

 ment of the new greenhouses. The shells were about four 

 feet below the surface, in a deposit of marl underlying two 

 feet of black, clayey soil. The fauna contains several species 

 which now have a more northern range, as Pisidium costatum, 

 P. tenissimum calcareum, Valvata sincera, and Galba obrussa 

 decampi, and there is reason to believe that the pond in which 

 these fossils lived occupied a kettle hole 011 the inner face 

 of the Champaign moraine when the ice of the late Wisconsin 

 glaciation was at or near Chicago. If this is so, then the 



