PLATE XXI. 



EXOGTTIA COSTATA VOT. CANCELLATA var. nOV. (p. 53). 



Figure 1. — Right or upper valve from the Selnia chalk (Exogyra costala zone); gullies near public road 3 J miles 

 northwest of Booneville, Prentiss County, Miss. (II. S. Geol. Survey coll. 6456). Shows the surface 

 characters of this valve. 

 2. — Left valve from impure phase of Selma chalk {Exogyra costala zone); Selmer-Adamsville road, 3 miles 

 west of Adanisville, McNairy County, Tenn. (U. S. Geol. Surv. coll. 6927). Shows absence of radiating 

 costse toward margin of a large specimen, and depressed area extending from back to lower posterior 

 margin. 



71 



