154 PALAEONTOLOGY OF ILLINOIS. 



lateral regions, and depressed in the middle towards the front, 

 so as to form a rather broad, rounded, moderately deep sinus, 

 occupied by from one to three obscure rounded plications, usu- 

 ally not extending up to the middle of the valve ; front curv- 

 ing down and more or less produced in the middle, in the form 

 of a semicircular or subquadrate projection, fitting into a cor- 

 responding sinus in the margin of the other valve; antero- 

 lateral margins each occupied by two obscure, usually short, 

 rounded plications; beak short, small, and closely incurved 

 upon that of the other valve. Surface apparently smooth, or 

 having only a few subimbricating marks of growth near the 

 front, but showing under a lens traces of exceedingly fine, 

 nearly obsolete, radiating striae, which are also visible on the 

 laminse of partly exfoliated specimens. Length (of a gibbous 

 specimen), 0.50 inch; breadth, 0.56 inch; convexity, 8.40 inch. 



This shell varies considerably in form, and in the distinctness of its plica- 

 tions, some individuals being much more gibbous than others; while in some 

 examples (particularly young shells) the plications are almost entirely obso- 

 lete, and in others more distinctly defined, and extending farther up towards 

 the beaks. The obscure, minute, radiating striae, mentioned above, seem to be 

 produced by the fibrous structure of the shell, rather than by true surface mark- 

 ings, since they are visible on all the laminae of exfoliating specimens. On 

 protected portions of well preserved specimens, we have also thought we could 

 sometimes see traces of minute, concentric striae, by the aid of a good lens. 



We are not quite sure Dr. Shumard is right in regarding figure 56 and 5 c, 

 plate C, of the Missouri report, cited above, as a variety of this species, since 

 we have not seen all the intermediate gradations connecting these forms. 



After a careful comparison of the shell under consideration, with a fine series 



of Rhynclwnella reniformis and R. pagnus, sent by Mr. Davidson, from the 



Carboniferous limestone of England and Ireland, it seems to us that it will be 



difficult, if not impossible, to distinguish it from certain varieties of both of 



these forms, which are themselves regarded by some authors as only varieties of 



one species. 



Locality and position : Rockford, Indiana, in the Goniatite bed of the Kin- 



derhook group, Subcarboniferous series; also at the same horizon at Chouteau 



Springs, and numerous other localities in Missouri. 



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