INVERTEBRATES. 127 



This ridge slopes abruptly to the dorsal margin, and more 

 gradually to the anterior portion of the shell. Posterior wing 

 equal to about two-fifths of the entire length of the shell. Um- 

 boiial region squarely truncated at the apex, upper portion of 

 the anterior margin nearly straight to the point where it joins 

 the ventral portion of the valve, or about one-fourth its entire 

 length. Surface ornamented with numerous concentric lines of 

 growth, which are closely arranged on the umbo, but become 

 more widely spaced on the lower margins of the shell. Right 

 valve unknown. 



Length 1'x inches: greatest breadth about % inch; convexity 

 of left valve % inch. 



Position and locality: Upper Coal Measures, LaSalle, 111. 



Collector, A. H. Wort hen. 



No. 2525 of the Illinois State Museum. 



GEMS MYALINA, DeKoninck. 

 MYALINA MONKOENSIS. Worthen. 



PL XX, Figs. 10-lOa. 



Myalina monroensis, Worthen, March, 18*1. Bulletin No. 2, of the Illinois State Museum 

 of Natural History, page 15. 



Shell small, nearly equi valve, moderately convex on the um- 

 bonal slopes from the beaks two-thirds the distance to the 

 posterior extremity, becoming gradually depressed below. Pos- 

 terior and postero-lateral regions cuneate; cardinal margin 

 nearly straight, and about half as long as the valves, gradually 

 rounding below to the basal extremity. Beaks small, slightly 

 oblique, and not projecting beyond the cardinal margin. Angle 

 of the anterior and posterior margin about 50. Surface smooth, 

 or with very obscure traces of lines of growth. Length of a 

 medium size specimen *- ](i inch; greatest width of the valve 7 /ie 

 inch, convexity of valve about % inch. 



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

 Monroe county. 111. 



Collector, A. H. Worthen. 



No. 2522 of the Illinois State Museum. 



