256 PALAEONTOLOGY OF ILLINOIS. 



LAMELLIBRANCHIATA. 



Genus AVICULOPECTEN, McCoy, 1851. 



(Ann. Mag. Nat. Hist., vol. vii, p. 171.) 



m 



Aviculopecten Oweni, M. and W. 



PI. 18, fig. 2 a, 2 b, 2 c. 



Aviculopecten Oweni, Meek and Worthen, Oct., 1860. Proceed. Acad. Nat. Sci., Phil- 

 adelphia, p. 452. 



Shell small, truncato-subcircular; base regularly rounded ; 

 posterior margin rounding from above the middle to the ventral 

 border; anterior side rounded below; hinge equaling about 

 two-thirds the diameter of the valves from the posterior to the 

 anterior side, ranging at right angles to longer axis* of the shell. 

 Left valve moderately convex; anterior ear compressed, sepa- 

 rated from the swell of the umbo by a rounded, shallow 

 depression, and defined by a shallow sinuosity in the anterior 

 margin ; having the form of an inequilateral triangle, the ante- 

 rior side of which is shorter than either of the others, very 

 slightly rounded at the nearly rectangular extremity; posterior 

 ear a little smaller than the other, compressed, very short, not 

 separated from the margin below by a distinct sinus, termina- 

 ting in an obtuse angle of about 100°; umbo rather convex, 

 located apparently a little behind the middle of the hinge ; 

 surface ornamented by numerous, fine, closely arranged, radia- 

 ting striae, which increase by implantation, and are crossed by 

 a few irregular marks of growth. (Right valve unknown.) 

 Diameter from the ventral margin to the hinge, 0.48 inch; do. 

 from the anterior to the posterior margin, 0.50 inch ; length 

 of hinge, 0.35 inch. Number of striae in 0.10 inch near the 

 ventral margin, about 13. 



Locality and position : Keokuk division, Subcarboniferous series, near War- 

 saw, Illinois. 



