144 PALAEONTOLOGY OF ILLINOIS. 



Greatest breadth of the aperture about 4 inches; height of 

 the same 2% inches; length from front to back 3 13 / 16 inches. 



This shell is only known from a cast in limestone, but it may 

 be readily distinguished by its size and general form from all 

 known American species. 



Position and locality : Lower Coal Measures, Monroe county, 

 Illinois. 



Collector, A. H. Worthen. 



No. 2543 of the Illinois State Museum. 



GENUS NATICOPSIS. McCoy. 

 NATICOPSIS MADISONENSIS. Worthen. 



PL XXV, Figs. 4,6. 



Naticopsis madisonensis, Worthen, March, 1884. Bulletin No. 2 of Ihe Illinois State 

 Museum of Natural History, page 9. 



Shell large, sub-rhomboidal or sub-rectangular in outline ; vol- 

 utions about three, the last one comprising more than two- 

 thirds the entire length of the shell, all rounded, and the last 

 quite gibbous; umbilicus deep, aperture apparently subcircular. 

 The specimen is a cast in limestone, and shows no external 

 markings except some faint lines of growth. 



Length 2 1 / 16 inches; breath of the last volution 1 5 / 16 inches. 



In form and size this shell approaches most nearly to N. 

 giganteus of the Coal Measures, but it differs from that in 

 having one less volution, and it also lacks the flattened depres- 

 sion on the outer volution which characterizes that species. 



Position and locality: St. Louis limestone, Madison county, 

 Illinois. 



Collector, A. H. Worthen. 



No. 2546 of the Illinois State Museum. 



GENUS POLYPHEMOPSIS. Portlock. 

 POLYPHEMOPSIS? KEOKUK. Worthen. 



PL XXIII, Fig. 3. 



Polvphernopsis? Keokuk, Worthen, March, 1884. Bulletin No. 2 of the Illinois State 

 Museum of Natural History, page 9. 



A cast of this shell in limestone from the lower portion of 

 the Keokuk limestone in the vicinity of Keokuk presents the 

 following characters : 



