124 PAL/EONTOLOGY OF ILLINOIS. 



and about equaling three-fourths the entire length of the valves; 

 basal margin slightly sinuous near the middle; anterior ex- 

 tremity broadly rounded and wider than the posterior; beaks 

 depressed and located very near the anterior extremity. An 

 ill-defined radiating ridge extends from behind the beaks 

 obliquely to the posterior margin, between which and the car- 

 dinal margin some indistinct traces of longitudinal costa 1 may 

 be seen. 



The specimen is a cast in limestone of both valves slightly 

 displaced. Minute surface markings unknown. 



Length 1 9 / 10 inches, height from beak to base % inch. 



Position and locality: Chester limestone, Chester, 111. 



Collector, A. H. Worthen. 



No. 2501 of the Illinois State Museum. 



PLEUROPHORUS MINIMUS. Worthen. 



PI. XX, Figs. 4-4a. 



Pleurophorun minimus, Worthen, March, 1884. Bulletin No. 2 of the Illinois State 

 Museum of Natural History, page 17. 



Shell small, about twice as long as high, moderately convex. 

 Outline elongate ovate. Anterior margin slightly produced 

 forming an obtuse point, posterior margin obliquely truncated ; 

 dorsal and ventral margins nearly straight and sub-parallel, 

 the latter rounding up to the obtuse anterior extremity. Beaks 

 depressed, and placed about one-fourth the length of the valve 

 behind the anterior extremity. 



Posterior umbonal slopes sub-angular from the beaks obliquely 

 backward and downwa-rd to the posterior extremity, with a 

 decided depression between their slopes and the cardinal mar- 

 gin. Surface marked by linear lines of growth, crossed by rad- 

 iating costs?, which are most strongly defined on the depression 

 of the cardinal area, and on the posterior portion of the shell. 



Length 5 / 16 inch ; height % inch; convexity of the valves about 

 Vie incn - 



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

 Monroe county, Illinois. 



Collector, A. H. Worthen. 



No. 2524 of the Illinois State Museum. 



