128 PALEONTOLOGY OF ILLINOIS. 



GENUS NUCULA, Lamarck. 

 NUCULA ILLINOIENSIS. Worthen. 



PL XIX, Figs. 4-4a. 



Xui'iilti Hlinoiensis, Worthen, March, 1884. Bulletin No. 2, of the Illinois State Museum 

 of Nalural History, page 15. 



Shell small, transversely ovate, depressed convex, the greatest 

 convexity about the middle of the valves ; slightly excavated in 

 front of the beaks ; anterior end regularly rounded to the ventral 

 margin, dorsal outline declining moderately from the beaks to 

 the posterior extremity ; ventral margin slightly curved ; beaks 

 depressed and situated about one-third the distance from the 

 anterior to the posterior extremity. 



Length 6 / 1(5 inch; height 4 /ie inch. Surface nearly smooth, but 

 in some of the specimens, faint lines of growth may be seen with 

 a good lens around the borders of the shell. 



Position and locality : Oolitic bed of the St. Louis limestone, 

 Monroe county, 111. 



Collector, A. H. Worthen. 



No. 2519 of the Illinois State Museum. 



GENUS AVICULOPINNA, Meek. 

 AVICULOPINNA ILLINOIENSIS. Worthen. 



PL XX, Figs. 5-5a. 



Aviculopinna iUinoiensis, Worthen. March, 1884. Bulletin No. 2, of the Illinois State 

 Museum of Natural History, page 13. 



Shell rather larger than the typical species, valves moderately 

 and about equally convex, cardinal and ventral margins curved, 

 and converging gradually from the base to the apex. Surface 

 ornamented with numerous thread-like lines or lamellae that 

 curve gracefully round to the dorsal margin, and are separated 

 liy rather broad spaces on the dorsal portion of the valve. The 

 apex is broken away so that its form cannot be determined. 



Length 1 9 / 10 indies; greatest width % inch; greatest thickness 

 % inch. This species differs from the A. americana, in the cur- 

 vature of its margins and the greater convexity of the valves. 



