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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 134

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[page]133.[/page] fattened. Hinge straight; posterior margin obliquely truncated; ventral margin nearly parallel to hinge; anterior margin sloping downward and forward from the beaks and curving rather abruptly into ventral margin. On each valve, between the raised hinge and an umbonal ridge parallel to the hinge, there is a narrow depressed area . Beside the ridge just mentioned there is another passing in an oblique curve from the beak to the posterior margin. Surface of both valves marked by fine lines of growth and by concentric wrinkles which become obsolete on the posterior umbonal ridges. Geol. Pos. Missouri stage, Western States. Ref.-- Final Rept. U. S. Geol. Survey of Nebraska, p.220.
 
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