vascular lines, the coarser ones of which branch from the pair of main 

 sinuses and are subjected to frequent subdivisions. 



Dimensions. The figured specimen of the cotype, preserved 

 as an internal cast upon which the internal structure is imprinted, 

 was obtained from the Harrison ore and shows the following measure- 

 ments: length 23 mm., width 24 mm., convexity of dorsal valve 5 mm. 

 (much flattened). The measurements of another cotype, which 

 shows the external characters of the shell and the normal convexity 

 are: length 22.5 mm., width 23 mm., convexity of dorsal valve 9 mm. 

 This species of Orbiculoidea was also formed at one locality in the 

 Sharon ore where it was the only fossil discovered. The size of the 

 Sharon specimens is on the average considerably smaller than that 

 of the forms from the Harrison ore. A large specimen from the Sharon 

 ore measures: length 17 mm., width 18 mm., convexity of dorsal 

 valve 4 mm. (somewhat compressed). 



Remarks. The distinctive features of this species are its circular 

 form, its inconspicuous, depressed beak which overhangs the posterior 

 margin, and its great convexity in the central part of the valve. It 

 resembles 0. meekana Whitfield in form, but the latter species has 

 the beak elevated, forming the point of greatest convexity of the 

 valve; it is also situated farther from the posterior margin, about 

 one-third the length of the shell. 0. munda (Miller and Gurley) 

 and 0. planidisca Raymond are forms which have the beak almost 

 marginal in position. The former is distinctly elliptical in form and 

 has a beak much less depressed and inconspicuous than the species 

 under consideration ; the greatest convexity also occurs in the posterior 

 third of the shell. 0. planidisca is also elliptical, the width being 

 only three-fourths as great as the length. 



Horizon and locality. Harrison ore: Jackson County (Locality 1), 

 c. Sharon ore: Glen Nell mine, Jackson County (Locality 5), r. The 

 specific name has been given in honor of Mr. Wilber Stout of the 

 Geological Survey of Ohio. 



Orbiculoidea capuliformis (McChesney) 



1860 Discina capuliforma. McChesney, Desc. New Pal. Fossils, p. 72. 



1861 Discina capuliformis. McChesney, Trans. Chicago Acad. Sci., Vol. I, p. 23, 



PI. 2, Fig. 20. 



Coal Measures: Springfield, Illinois; 12 miles northwest of Richmond, 



Missouri. 



Description. Specimens of an Orbiculoidea are abundant in the 

 black bone shale on the Sharon ore horizon and have been referred 

 with some doubt to 0. capuliformis. They show considerable varia- 

 tion in size, ranging from those only 6 mm. in diameter to forms which 

 are relatively large; the outline of both valves is circular. The beak 

 of the dorsal valve is slightly posterior to the center; as the specimens 

 are greatly flattened, it is possible only to make conjectures concern- 

 ing the convexity, which seems to have been moderate. The ventral 



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