MOLLUSCA. 457 



1852. Exogyra costata Roemer, Kreide von Texas, p. 72. 

 1854. Exogyra costata Con., Rep. U. S. and Mex. Bound. 



Surv., vol. i, pt. 2, p. 154; pi. 8, fig. 3; pi. 9, figs. 



1-2; pi. 10, fig. i. 

 1858. Exogyra costata Emm., Rep. N. Car. Geol. Surv., p. 278, 



fig. A. 



1860. Exogyra costata Owen, 2nd Rep. Geol. Reconn. Ark., pi. 



7, % 4- 



1 86 1. Exogyra tostata Gabb, Synop. Moll. Cret. Form., p. 178. 

 1864. Exogyra costata Meek, Check List Inv. Foss. N. A., Cret. 



and Jur., p. 6. 

 1868. Exogyra costata Con., Cook's Geol. N. J., p. 374, fig. i, 



p. 724. 

 1876. Exogyra costata Gabb, Proc. Acad. Nat. Sci. Phil., 1876, 



P- 323- 

 1884. Exogyra costata White, 4th Ann. Rep. U. S. G. S., p. 



304, pi. 51, figs. 1-2, pi. 57, figs. 1-2. 

 1886. Exogyra costata Whitf., Pal. N. J., vol. i (Monog. U. S. 



G. S., vol. 18), p. 39 (in part). 

 1896. Exogyra costata Say, Bull. Am. Pal., vol. i, p. 291 (No. 



5, P- 21). 



1902. Exogyra costata Hill and Vaughan, U. S. G. S., Geol. 

 Atlas, Austin Folio, fig. 52. 



Description. "Shell large, thick, and ponderous, irregularly 

 circular or subovate in outline, plano-convex in profile and 

 obliquely coiled at the apex, the lower valve sometimes showing 

 one and a half to nearly two volutions in well-preserved adult 

 specimens. Convex valve deep, and on the back strongly an- 

 gular, especially so in the earlier parts formed, the portion near 

 the beak often being sharply carinate and smooth. Surface of 

 the valve marked by strong radiating costse, which are round 

 on the surface, and separated by narrow depressions. Costse 

 frequently bifurcating, and radiating from or dividing along 

 the umbonal ridge ; one set curving toward the anterior side and 

 the other toward the basal margin. Upper or left valve flat or 

 slightly convex, often becoming slightly concave toward the 

 antero-basal margin in advanced stages of growth, while in 



