INVERTEBRATES. 125 



PL,ETJROPHORUS MONROENSIS. Worthen. 



PI. XVIII, Fig. 3-3a. 



Pleurophorus monroensis, Worthen, Marsh, 1884. Bulletin No. 2, of the Illinois State 

 Museum of Natural History, page 17. 



Shell elongate-oblong, moderate!}' convex ; umbonal ridges 

 forming- 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 an- 

 terior extremity, surface smooth. 



Length % inch; height % inch; convexity about % inch. 



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

 Monroe county, 111. 



Collector. A. H. Worthen. 



No. 2523 of the Illinois State Museum. 



GENUS MODIOLA, Lamarck. 

 MODIOLA ILLINOIEXSIS. Worthen. 



PL XX, Fig. 2-2a. 



Modiola HUnoirnsis, Worthen, March, 1884. Bulletin No. 2 of the Illinois State Museum 

 ot Natural History, page 16. 



Shell below medium size, length from the anterior to the pos- 

 terior extremities a little more than twice the width of the 

 valves, narrowing rather rapidly behind the posterior extremity 

 of the hinge line; valves strongly convex in the umbonal region, 

 and obliquely across the valve to the ventral margin, com- 

 pressed behind; beaks depressed, hinge line equal to three-fifths 

 the entire length. A shallow depression extends from the beak 

 obliquely to the ventral margin in front of the umbonal ridge 

 posterior margin narrowly rounded, anterior extremity obtusely 

 pointed, ventral margin slightly sinuate in the middle, hinge 

 line straight, surface nearly smooth but showing a few obscure 

 lines of growth. 



