INVERTEBRATES. 123 



GENUS MACRODON, Lycett. 

 MACRODON SANGAMONENSIS. (sp. nov.) 



PL XXI, Figs. 3-3a. 



Shell large, transversely elongated, hinge line equal to about 

 4 /s of the entire length of the shell. 



Posterior margin compressed and obliquely truncated, so as 

 to meet the hinge line at a rather acute angle; posterior ex- 

 tremity quite narrow, and rounding gently downward to the 

 basal margin. 



Anterior margin regularly rounded from the anterior extrem- 

 ity to the basal margin, which is slightly sinuous about the 

 middle. 



Beak depressed, strongly incurved, and placed about one-fifth 

 the length of the hinge line from the anterior extremity. 



A gradually widening depression extends from the beaks to 

 the posterior extremity on the dorsal margin, and on this flat- 

 tened portion six or seven strong striae may be seen, which ex- 

 tend from the beak to the posterior extremity. Strong lines of 

 growth extend around the basal margin, and minute transverse 

 striae are visible under a lens, especially on the anterior portion 

 of the shell. 



Length 2% inches; length of hinge 1% inches: height 15 / 1( . inch; 

 convexity of valves 7 / l6 inch. 



Position and locality: Coal Measures, Rolls Ford, Sangamon 

 county, 111. 



Collector, A. H. Worthen. 



No. 2585 of the Illinois State Museum. 



GENUS PLEUROPHORITS, King. 

 PLEUROPHCRUS CHESTERENSIS. Worthen. 



ri. XX, Fig. C. 



Pteurophoma chesterensis, Worthen, March, 1884. Bulletin No. 2 of the Illinois State 

 Museum of Natural History, page 16. 



Shell of medium size, nearly cylindrical when the valves are 

 closed, almost three times as long as high, most strongly con- 

 vex on the anterior half of tlic shell; cardinal margin straight 



