INVERTEBRATES. 113 



Length from beak to base 15 / 16 inch; breadth 1 inch; length 

 of hinge line about % inch. This shell may be readily distin- 

 guished from otlu-i-.- <>r.urring in the same horizon by its broad 

 rounded cost*. 



Named in honor of my Mend Prof. Orestes St. John, of Topeka, 



Kan- 



Position and locality: Keokuk limestone, Warsaw, 111. 



Collector, A. H. Worthen. 



No. 2504 of the Illinois State Museum. 



A vii TLOPECTEN NiOTENSis. Worthen. 



PL XXH. Fig. 10. 



Acteidopecten niotense Worthen, March, 18&4, Bulletin No. 2, of the Illinois State Museum 

 of Natural History, page 19. 



Shell large, orbicular, nearly or quite equilateral, length and 

 breadth about equal, hinge line shorter than the greatest width 

 of the shell. Left valve moderately convex, basal margin 

 broadly and evenly rounded, anterior ear short, triangular, and 

 separated from the lateral margin by a well-defined sinus. Pos- 

 terior ear more than twice as long as the anterior, and pointed 

 at the extremity. Surface of the valve ornamented with numer- 

 ous fine radiating cost*?, of which about fourteen may be 

 counted near the basal margin in the space of half an inch. 

 These cost* appear to become obsolete on the ears and on the 

 lateral borders of the shell. 



Length 2;\ inches; breadth about the same; length of hinge 

 line about 2 inches. 



This shell may be readily distinguished from the .1. vansovi- 

 ensis. described and figured in Vol. VII of these reports, by the 

 much finer radiating cost* with which it is ornamented. 



Position and locality: Keokuk limestone, Niota, Hancock 

 county, Illinois. 



Collector, A. H. Worthen. 



No. 2505 of the Illinois State Museum. 



