328 PALEONTOLOGY OF ILLINOIS. 



forwards and upwards above. Left valve much compressed ; 

 anterior ear rather small, triangular, flattened and distinct 

 from the umbonal slope. Posterior ear of about the same size 

 as the other, flattened and terminating in a rather acute angle ; 

 separated from the posterior margin by a broad, very shallow, 

 rounded sinus. Beak compressed, located very nearly at the 

 middle of the hinge. Surface ornamented by very fine, radia- 

 ting, thread-like striae, which increase by intercalation and are 

 generally rather smaller than the depressions between ; crossing 

 these are equally fine, regular, less distinct concentric striae. 

 (Right valve unknown.) Diameter from the ventral margin 

 to the hinge, 0.36 inch ; breadth from the anterior to the pos- 

 terior margin, 0.35 ; length of hinge, 0.23 inch ; number of 

 radiating striae in 0.10 inch at the ventral margin, about 12. 



This delicate little shell will be readily distinguished from the last by its 

 smaller size and much finer surface markings, which form a neat cancellated 

 style of ornament. It is also a more oblique shell, with a rather straighter and 

 proportionally shorter hinge, and more angular anterior ear. 



Locality and position : Same as preceding. 



Aviculopecten Koninckii, M. and W. 



PI. 26, fig. 8. 



Aviculopecten Koninckii, Meek and Worthen, Oct., 1860. Proceed. Acad. Nat. Sci., 

 Philad., p. 453. 



Shell large, thick, truncato-subcircular, rather compressed, 

 wider than long, not oblique, broadly rounded on the pallial 

 margin, and more narrowly rounded on each side below the 

 ears; hinge straight, apparently rather less than the greatest 

 breadth of the valves below, and ranging very nearly at right 

 angles to the vertical axis of the shell ; right valve compressed, 

 most convex near the umbo, and flattened towards the free 

 margins; anterior ear having the form of an inequilateral tri- 

 angle, the anterior side being shorter than either of the others. 



