DESCRIPTION OF SPECIES 177 



1852 Orthis arachnoidea. 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: Leaven worth, Kansas. 

 1892 Derbya crassa. Hall and Clarke, Pal. N. Y., Vol. 8, Pt. 1, PI. 10, Figs. 10, 11; 



PI. 11 A, 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 col- 

 lections 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, and it occurs with very few exceptions at every 

 locality from which fossils were collected. The size is somewhat vari- 

 able, 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. Dorsal valves, when 

 uncrushed, are quite convex, while ventral valves are slightly convex 

 in the umbonal region, becoming flattened toward the margins. The 

 surface is characteristically covered with radiating lines, a coarse one 

 alternating with one, 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. 



Horizon and locality. Very abundant and widely distributed in 

 the middle and upper Pottsville formation including the following 

 members: Lowellville, Boggs, Lower Mercer limestone, Lower Mercer 

 ore, Upper Mercer, Me Arthur, and Black Flint, aa. 



Derbya robusta (Hall) 

 PI. VII, fig. 11 



1858 Orthis robusta. Hall, Geol. Iowa, Vol. I, Pt. 2, p. 713, PI. 28, Figs. 5a-c. 

 Lower Coal Measures : St. Clair County, Illinois. 



Description. This large brachiopod is confined to the Lower 

 Mercer horizon in its occurrence in the Pottsville formation of this 

 State; it is by no means a common or characteristic fossil except at a 

 single locality in Vernon Township, Scioto County. A considerable 

 number of well-preserved specimens of both dorsal and ventral valves 

 which agree rather closely with Hall's species, have been examined. 

 However, our shell differs from Hall's figures in having the hinge line 

 considerably shorter, so that its length is less than the greatest width of 

 the shell below. As this group of brachiopods, however, is a very 

 variable one, the difference in the length of the hinge line is not thought 

 to be of specific value. In Vernon Township, Scioto County, it is asso- 

 ciated with Chonetes choteauensis, Aulacorhynchus millepunctatus, and 



