Horizon and locality. Lowellville member: Poverty Run, Mus- 

 kingum County (Locality 19), c. Boggs limestone: Blunt Run, 

 Muskingum County (Locality 27), r. 



Genus Rhipidomella Oehlert 

 Rhipidomella pecosi (Marcou) 



1859 Orthis pecosi. Marcou, Geol. N. Am., p. 48, PI. 6, Fig. 14. 

 Mountain limestone: Pecos Village, New Mexico. 



This small shell with its almost circular form and fine radiating 

 lines is by no means common among the Pottsville fauna of Ohio, 

 but it is found most often in the Lower Mercer limestone. In the 

 Lowellville member it was represented only by a single ventral valve 

 which measures: length 6.5 mm., width 7.7 mm., convexity 2 mm. 



Horizon and locality. Lowellville member: Poverty Run, Mus- 

 kingum County (Locality 19), r. Lower Mercer and upper Potts- 

 ville members. 



Genus Derbya Waagen 

 Derbya crassa (Meek and Hayden) 



1852 Orthis umbraculum ? Hall, Stansb. Exped. Gt. Salt Lake, p. 412, PI. 3, Fig. 6. 



Carboniferous: Missouri River, above Fort Leavenworth. 

 1852 Orthis umbraculum ? Owen, Geol. Rep. Wis., Iowa and Minn., PI. 5, Fig. 11. 



Carboniferous: Missouri River, near mouth of Keg Creek, and at Council 



Bluffs. 

 1852 Orthis arachoidea. Roemer, Kreid. von Texas, p. 89, taf. 1, Figs. 9a, b. 



Carboniferous: San Saba Valley, Texas. 

 1858 Orthisina crassa. Meek and Hayden, Proc. Acad. Nat. Sci. Phil., p. 261. 



Coal Measures: Leavenworth, Kansas. 

 1892 Derbya crassa. Hall and Clarke, Pal. N. Y., Vol. 8, Pt. I, PI. 10, Figs. 10, 11; 



PI. 11A, Figs. 28-33; PI. 11B, Figs. 23, 24; PI. 20, Figs. 12, 13. 



Upper Coal Measures: Near Kansas City, Missouri, and Winterset, Iowa. 



Description. Among the Pottsville fossils at hand are large 

 collections of Derbya crassa, which is one of the most abundant, if 

 not the most abundant, fossil of the entire formation. Although 

 it has not been found below the Lowellville member, it is present 

 in all of the marine limestones where it occurs with very few excep- 

 tions at every locality from which fossils were collected. The size 

 is somewhat variable, but the measurements of an individual of average 

 size from the Lowellville black shale on Poverty Run are: length 

 20.5 mm., length of hinge line 20 mm., maximum width 23.5 mm. 

 The dorsal valves, when uncrushed, are quite convex, while the cen- 

 tral valves are slightly convex in the umbonal region, becoming flat- 

 tened toward the margins. The surface is characteristically covered 

 with radiating lines, a coarse one alternating with two or three finer 

 ones; crossed by fine, closely arranged, concentric lines which give 

 a crenulated appearance to the shell, and also by a few coarse lines 

 of growth. 



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