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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 30

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29. blunt incurved. Ventral valve having a deep sinus which in the umbowal region is unornamented, but in the anterior portion is marked by three plications. Beak closely incurved projecting generally over the hinge line, foramen broadly triangular. Surface marked, on both sides of the mesial fold and sinus, by from seven to twice as many plications. The two plications immediately contiguous to the sinus and fold, on both sides are formed by the bifurcation of single plications on the umbo. Lines of growth well defined around the anterior and lateral margins. Geological Position.- Keokuk and Saint Louis Limestone.
 
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