ORBICULOIDEA 41 



ORBICULOIDEA KEOKUK (Gurley) 



Plate I, Figs. 14-17 

 1884. Discina Keokuk Gurley, New Carboniferous Foss., ; Bull. No. 2, p. 6. 



Description. Shell rather large for the genus, subcircular in outline. 

 The dimensions of a pedicle valve, one of the cotypes, are: length 16.5 

 ram., width 18 mm. The width of a second cotype is 21 mm. 



Pedicle valve nearly flat, the pedicle opening excentric, its external 

 surface marked by regular, sublamellose, concentric lines of growth. 



Brachial valve depressed conical, the apex situated at about one-fourth 

 the length of the shell from the anterior margin, the surface apparently 

 gently convex from the apex to the anterior and lateral margins and 

 slightly concave to the posterior margin. The surface marked by fine, 

 crowded, rounded, raised, concentric lines of growth which are apparently 

 somewhat irregular in their development and distribution. Internally a 

 slight median ridge extends anteriorly from the apex for about one-third 

 the distance to the anterior margin of the valve. 



Remarks. The above description is based upon the two cotypes of the 

 species from Crawfordsville, Indiana. The brachial valve is incomplete 

 posteriorly so that the exact position of its apex, relative to the posterior 

 margin, cannot be determined. The length of the valve is also uncertain, 

 and, since it is more or less crushed, the depth, as well as the contour of 

 the sloping surface from the apex to margins, cannot be accurately deter- 

 mined. The shell substance is largely exfoliated, but enough is preserved 

 to show the character of the surface markings. The species is character- 

 ized by its nearly circular outline and the nature of its surface markings. 



Horizon. Keokuk (Crawfordsville shale). 



ORBICULOIDEA VARSOVIENSIS (Worthen) 

 Plate I, Figs. 23, 24 



1884. Discina varsoviensis Worthen, Bull. No. 2, 111. State Mus. Nat. Hist., 



p. 23. 



1890. Discina varsoviensis Worthen, Geol. Surv. 111., vol. 8, p. 102, pi. 11, 

 fig. 7. 



Description. Shell rather large for the genus, subelliptical in outline. 

 The dimensions of the holotype are : greater diameter 19.5 mm., lesser 

 diameter 16.1 mm. 



Pedicle valve nearly flat; the pedicle opening situated excentrically 

 in the line of the longer axis of the valve, and extending from the center 

 of the valve half way to its posterior margin, its lateral margins elevated 

 upon the internal surface of the valve to form an elongate, subovate 

 prominence which is probably a depression or excavation externally. 

 The external surface marked by sublamellose, concentric lines of growth, 

 from two to three of which occupy the space of 1 mm. 



