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Coal Measures and Coal Mining in Iowa, including paleontology and a discussion on the coal formation; also the methods of mining by Russell T. Hartman, 1898

Coal Measures and Coal Mining in Iowa by Russell T. Hartman, 1898, Page 77

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[page]76.[/page] Reference.-- Final Rept. U.S. Geol. Survey of Nebraska, page 174. [underlined]Derbya robusta[/underlined], Plate VI. figure 2. Orthis robusta, Hall, 1858, Geol. of Iowa Vol. I., Part II., p. 713. Derbya robusta, Hall, 1892, Paleontology of New York Vol VIII., part I. Shell large subelliptical in outline, hinge line straight, cardinal angles compressed. Shell plano-convex or concavo-convex. Dorsal valve gibbous, beak rudimentary, hinge area linear. Ventral valve flat or slightly concave with beak and umbo raised; beak prominent not incurved; hinge area moderately wide, broadly triangular; foramen triangular but closed nearly to base by deltidium. Interior:-- mesial septum about
 
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