GEOLOGY OF IvA SALLE COUNTY. 



they may be subsequently modified by change of the 

 conditions. The family was formerly called Melanians 

 from their supposed affinity to Oriental forms, but our 

 species have a plain or entire margin to their mantle. 

 The Oriental species have a fringed mantle. The soft 

 parts have not, as yet, been much studied in this coun- 

 try, and species, genera, etc., are based upon the char- 

 acters of the shell. We have four species, three of 

 them abundant in our rivers. 



GENUS PLEUROCERA, RAFlNEsQUE. — PLEUROCERA 

 SUBLUARE, LEA — (TRYPANOSTOMA.) 



The following description is from American Jour- 

 nal of Conchology, Vol. 1: "Acutely turrited, rather 

 thin, spire much elevated, apex acute, whirls twelve, 

 flat, carinate at apex, body whole, ang-ulate on the 

 periphery. Horn color, g-enerally light j^ellow and 

 bluish ash below the suture." 



In some specimens from one locality the whirls are 

 rounded, have from one to two bands, and are of a dark 

 horn color. From another, the whirls are eleven, the 

 color very light, resembling P. pallidum. Lea. I refer 

 to mature shells. In the young the diiferences are not 

 so marked. 



Pleurocera lewisii, Lea. — Sulcate, somewhat thin, 

 conical, elevated, spire much elevated, with indistinct 

 suture, whirls eleven, flattened, covered with sulca- 

 tions, of which there are four to five on the body, 

 w^hich is angulate on the periphery. Horn color or 

 dark brown, banded or without bands, white or purple 

 w^ithin. 



This species is found in the same localities as the 

 preceding one. P. Lewisii is a much broader shell at 

 the base, more pyramidal in shape, has sulcations, and 

 is of more solid texture than P. Subulare. Of the 



