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Family DISCINID-ffi. 



DISCINA ILLINOISENSIS, n. Sp. 



Plate VII, Fig. 2, dorsal view; Fig. 3, interior of ventral 

 valve; Fig. 4, dorsal valve ol another specimen; Fig. 

 5, interior of dorsal valve of another specimen. 



Shell large, subcircular in marginal outline, or slightly oval, 

 longitudinally; height nearly or quite equal to one-half the 

 diameter. Apex prominent, acute, and situated centrally or 

 slightly posterior to the central part of the shell, and moder- 

 ately incurved. The sides slope rapidly from the apex for one- 

 third the height of the shell and then more and more gently 

 to the margin, though, on the anterior side the shell is more 

 compressed toward the margin than it is on the posterior part. 



Surface marked by irregular radiating lines or striations, and 

 by strong concentric elevated lines. The striae and elevated 

 concentric lines become more and more conspicuous toward the 

 margin. The lower valve shows a depression on the side of the 

 foramen, but our specimen, as shown by figure 3, is somewhat 

 broken and pressed out of shape. The concentric lines are 

 strongly impressed upon it. 



The two specimens shown by figures 2 and 3 are from the 

 Coal Measures, in Knox county, Illinois, and the two specimens 

 shown by figures 4 and 5 are from Peoria county, Illinois. We 

 refer them all to the same species, though we look upon figure 

 2 as the best preserved and most typical. They are all in the 

 collection of Wm. F. E. Gurley. Figure 5 shows the interior of 

 the upper valve and where the shell is abraded the concentric 

 lines are strongly marked in the stone. 



This species is*so far removed from Discina nitida, that no 

 one would be enlightened by making a comparison with it. It 

 is more nearly related to Discina convexa, described from the 

 Upper Coal Measures, in the valley of the Verdigras river, in 



