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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 24

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23. is subquadrate, hinge line straight, cardinal angle articulate, compressed, no hinge area. Dorsal valve deeply concave. Ventral valve extremely convex, the greatest convexity being posterior to the middle; beak closely incurved and projecting beyond the hinge line. Surface marked by coarse bifurcating striae. The ventral surface is marked posteriorly by more or less prominent concentric wrinkles. Surface set with tubercles like [spine?] bases which [are the?] more numerous on the cardinal angles and about the lateral and posterior borders. Geol. positions.- Saint Louis Stage Localities; Saint Louis, Missouri; Milan, Illinois; Fairfield and Mount Pleasant, Iowa.
 
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