THE NAUTILUS. 

 A NEW PLANORBIS FROM ILLINOIS/ 



BY FRANK C. BAKER. 



While making a study of the larger Planorbi/< of the Big Ver- 

 milion River, Illinois, for a paper on the distribution of the 

 mollusks of that stream, it became apparent that two forms 

 were included under trivolnx which were quite separable. One 

 of these is the large, wide form to which Say gave the name 

 t.rivolvix. The other is a narrower form which the writer and 

 others have been calling glubratus (see Baker, Cat. 111. Moll., 

 p. 106) but which is not the true ghibrvttt* of Say, which, ac- 

 cording to Walker (Synopsis, p. 99), does not range outside of 

 the State of Florida. Say credited his original specimens to 

 Charleston, S. C. The new Illinois form may be characterized 

 as follows: 



Planorbis pseudotrivohis n. sp. 



Shell sinistral, whorls 5; body whorl rounded above and be- 

 low, the inner whorls carinated on both spire and umbilical 

 region; the spire whorls are very flat and slightly concave; the 

 earlier whorls are coiled so that they form a union with the 

 carina of the preceding whorl but the last whorl gradually 

 divides from this line, leaving a V-shaped depression between 

 the dorsal carina and the body whorl; this condition is uniform 

 for the dozen or so specimens examined: the base or umbilical 

 region exhibits three full whorls to the umbilicus; aperture 

 somewhat lenticular, rounded above and below, sometimes a 

 trifle expanded, and bordered with red; color of shell yellowish 

 or corneous inclining to brown; surface notably shining. 



Height, 9; greatest diameter, 20.5; aperture height, 8; breadth, 

 9 mm. Holotype. 



Height, 9; greatest diameter, 19; aperture height, 8; breadth, 

 9 mm. Paratype, 5 whorls. 



Height, 6; greatest diameter, 11.5; aperture height, 5; breadth, 

 o mm. Immature, 4 whorls. 



* Contribution from the Museum of Natural History, University of Illinois, 



No. 8. 



