BULLETIN NO. 2. 15 



terior margin nearly straight to the point where it joins the ven- 

 tral 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. Eight valve unknown. 



Length 1| inches ; greatest breadth about h inch ; convexity of 

 left valve ^ inch. 



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



No. '2.s25 of the Illinois State collection. 



Collector, A. H. Worthen. 



GENUS MYALINA, DeKoninck. 



MYALINA MONKOENSIS. (sp. nOV.) 



Shell small, nearly equivalve, moderately convex on the umbonal 

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

 extremity, becoming gradually depressed below. Posterior 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 

 11 inch ; greatest width of the valve f^ inch, convexity of valve 

 about ^ inch. 



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

 Monroe county. 111. 



No. 2522 of the Illinois State collection. 



Collector, A. H. Worthen. 



GENUS NUCULA, Lamark. 



NUCULA ILLINOIENSIS. (sp. nOV.) 



Shell small, transversely ovate, depressed convex, the greatest 

 convexity about the middle of the valves ; slightly excavated in 

 front of the beaks ; anterior end regularly rounded to the ventral 

 margin, dorsal outline declining moderately from the beaks to the 

 posterior extremity ; ventral margin slightly curved ; beaks depress- 

 ed and situated about one-third the distance from the anterior to 

 the posterior extremity. 



Length -^-^ inch ; height fV inch. Surface nearly smooth, but 

 in some of the specimens, faint lines of growth may be seen with 

 a good lens around the borders of the shell. 



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

 Monroe county. 111. 



No. 2519 of the Illinois State collection. 



Collector, A. H. Worthen. 



