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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 79

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[page]78.[/page] 1867, Trans. Chicago Acad. Sci., Vol. I p.120. In outline the shell is sub-trigonal, double convex, becoming globose with age; sometimes wider than long, at other times longer than wide; length of hinge line one third to one half the greatest width of shell. Dorsal valve generally a little more gibbous than the ventral valve; umbo prominent, rounded, projecting back of hinge line; beak obscure; posterior lateral margins compressed; hinge area linear. Ventral valve convex; beak sharp, projecting beyond the posterior part of the other valve; hinge area triangular, wide, striated, plane, or concave, foramen closed. Surface of both valves marked by rather large plications and by beautiful radial striae, also by zigzag
 
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