PLATE XX. 



Exogyka costata Say (p. 50'). 



Figure 1. Specimen with valves attached, from the Ripley formation (Exogyra costata zone); Chattahoochee River, 

 opposite the mouth of Burstahatchee Creek, 5V miles above Eufaula, Ala. (U. S. Geol. Survey coll. 6398). 

 Shows right or upper valve with costse more strongly developed than normal, and costae toward apical 

 portion of left valve. 



Exogyka costata var. cancellata var. nov. (p. 53). 



Figure 2. Left or lower valve from the Selma chalk {Exogyra costata zone); West Point-Cedar Bluff road, 1 mile east 

 of Cedar Bluff, near west end of bridge over Line Creek, Clay County, Miss. (U. S. Geol. Survey coll. 

 6862). Shows characteristic ornamentation of surface. 



3. Left valve from impure phase of the Selma chalk (toward base of Exogyra costata zone); cut of Southern 



Railway 2 J miles west of Corinth, Alcorn County, Miss. (U. S. Geol. Survey coll. 6879). Shows character 

 of ornamentation on apical portion of shell. 



4. Left valve from the Selma chalk (Exogyra costata zone); exposure in public road, 3 miles northwest of 



Macon, Noxubee County, Miss. (U. S. Geol. Survey coll. 6S40). Shows extreme development of con- 

 centric ridges. 



