PLATE XVIII. 



Exogtra costata Say (p. 50). 



Ponderous left valve from the Selma chalk I Eiogyra costata zone); 6 miles north of Scooba, on Wahalak-Biiinsvillp 

 road, westward-facing slope of Wahalak Creek valley, Kemper County. Miss. (U. S. Geol. Survey coll. 6480rfi. 

 Shows the absence or faint development of costee toward the margin of a large overgrown specimen. 



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