132 PALEONTOLOGY OF ILLINOIS. 



to the opposite margins, extremities rounded ; beaks depressed, 

 oblique, and located about one-sixth the entire length of the 

 valves behind the anterior extremity. 



Surface smooth or showing only a few obscure lines of growth. 



Length of the largest specimen obtained 13 /i e inch; height 

 from beak to base 6 / 16 inch. 



Position and locality : Obtained from the oolitic beds of the 

 St. Louis limestone 011 Fountain Creek, Monroe county, 111. 



Collector, A. H. Worthen. 



No. 2515 of the Illinois State Museum. 



SOLENOMYA? iGWENSis. Worthen. 



PL XIX, Fig. 5-5a. 



Solenomya? lowensis, Worthen, March, 1884. Bulletin No. 2 of the Illinois State 

 Museum of Natural History, page 13. 



Shell small, narrow, oblong, valves apparently closed at each 

 extremity. 



Dorsal and ventral margins nearly parallel, beaks depressed 

 and situated close to the anterior extremity. Surface markings 

 unknown. 



Length of the largest specimen 7 /io inch; height \ inch; 

 thickness % inch. This little shell is only known from a few 

 casts obtained from the shaly layers of the St. Louis limestone 

 near Pella, Iowa, where it was associated with Allorisma, marion- 

 ensis, RhynchoneUa ottumwa, Spiriier littoni and Zaphrentis sp? 



Collector, A. H. Worthen. 



No. 2517 of the Illinois State Museum. 



GENUS ALLORISMA, King. 

 ALLORISMA ILLINOIENSIS. Worthen. 



PL XVin, Pig. l-la. 



Allorisma illinoiensis, Worthen, March, 1884. Bulletin No. 2 of the Illinois State Mus- 

 eum of Natural History, page 11. 



Shell large, elongate-ovate, length a little more than twice the 

 height; valves moderately convex, extremities broadly rounded; 

 dorsal margin straight and nearly parallel with the base; an- 

 terior extremity short and rather more narrowly rounded than 

 the posterior, and descending obliquely from the beaks. Beaks 



