23 



ture of the shell ; aperture sub-quadrangular, the pillar side 

 shorter ; the outer side, from the periphery to the angle 

 bordering the umbilical region, nearly straight, and equal to 

 the space from the periphery to the suture. 



Diameter *17; height '10 to *11 of an inch. 



Locality. — Piasa creek, above Alton, 111. 



^Pleurotomaria rotundata. — Shell sub-globose ; volu- 

 tions about five or six, convex, the last one very rotund or 

 ventricose ; suture distinctly marked, and the volution de- 

 pressed just below it, and rising in an obtuse, undefined an- 

 gle, below which is a distinct depressed revolving line, and 

 below this again a similar sub-angular elevation, which forms 

 the upper limit of the broad periphery of the outer volu- 

 tion ; thus making the upper side of the volution obscurely 

 biangular with one depression between the angles, and the 

 other towards the suture. [These angles and the depression 

 between are distinctly visible in the cast.] Aperture broad- 

 ly ovate ; umbilicus small ; surface marked by fine, closely 

 arranged revolving striae. 



Diameter *09 to '45 ; height '04 to '38 of an inch. 



Locality. — Spergen Hill and Bloomington, la. ; Alton, 111. 



'-/Pleurotomaria Wortheni. — Shell depressed sub-globose ; 

 spire but little elevated, oblique from the great expansion of 

 the last volution ; volutions about three, somewhat flattened 

 above, rapidly expanding, so that the last volution makes 

 nearly the whole bulk of the shell ; obtusely angulate on the 

 periphery ; upper margin of the volutions marked by a 

 row of strong nodes, which extend about one-third across; 

 surface marked above by striae parallel to the lines of growth 

 which on the last volution disappear in passing over the an- 

 gulate periphery ; base of last volution marked by strong 

 revolving lines on the space between the outer margin and 

 the umbilical area ; base deeply excavated about the umbili- 

 cal region, but the umbilicus is unknown. Aperture sub- 



