BULLETIN NO. 2. 17 



on the anterior half of the shell; cardinal margin straight and 

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

 margin slightly sinuous near the middle ; anterior extremity 

 broadly rounded and wider than the posterior ; beaks depressed 

 and located very near the anterior extremity. An ill -defined radi- 

 ating ridge extends from behind the beaks obliquely to the pos- 

 terior margin, between which and the cardinal margin some in- 

 distinct traces of longitudinal costfe may be seen. 



The specimen is a cast in limestone of both valves slightly dis- 

 placed. Minute surface markings unknown. 



Length Ij^ inches, height from beak to base | inch. 



Position and locality : Chester limestone, Chester, 111. 



No. 2501 of the Illinois State collection. 



Collector, A. H. Worthen. 



pleuroph:rus minima, (sp. nov.) 



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

 Outline elongate ovate. Anterior margin slightly produced form- 

 ing 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-angalar from the beaks obliquely 

 backward and downward to the posterior extremity, with a decided 

 depression between their slopes and the cardinal margin. Sur- 

 face marked by linear lines of growth, crossed by radiating costae, 

 which are most strongly defined on the depression of the cardinal 

 area, and on the posterior portion of the shell. 



Length fV i^ich ; height | inch ; convexity of the valves about 

 tV inch. 



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

 Monroe county, 111. 



No. 2524 of the Illinois State collection. 



Collector, A. H. Worthen. 



PLEUROPHORUS MONROENSIS. (sp. nOV.) 



Shell elongate-oblong, moderately convex ; umbonal ridges form- 

 ing the most convex part of the valves ; length about twice as 

 much as the height ; a well-defined ridge commencing at the beak 

 extends diagonally to the posterior extremity ; anterior ventral 

 region depressed ; cardinal margin straight, and rather more than 

 half the entire length of the shell ; basal margin nearly straight 

 in the middle, but rounding up in front to the anterior extremity ; 

 beaks depressed to the hinge line, and placed about one-fourth 

 the length of the shell behind the anterior extremity, surface 

 smooth. 



Length ^ inch ; height I inch ; convexity about ^ inch. 



