EXOGYHA COSTATA. 51 



1884. Exogyra costata White, Fourth Ann. Rept. U. S. Geol. Survey, p. 304, PL LI, figs. 1-2, PL LVTI, figs. 1-2. 



1885. Exogyra costata Whitfield (in part), Mon. U. S. Geol. Survey, vol. 9, 1886, pp. 39-11, PL VI, figs. 1, 2. Also 



Paleontology of New Jersey, vol. 1, 1886, pp. 39-41, PL VI, figs. 1, 2. . 

 1896. Exogyra costata Say, Bull. Am. Paleontology, vol. 1, p. 291 (No. 5, p. 21) (a reprint). 



1901. Exogyra costata Hill, Twenty-first Ann. Rept. U. S. Geol. Survey, pt. 7, PL XLVII, figs. 1, la. 



1902. Exogyra costata Hill and Vaughan, Austin folio (No. 76) Geol. Atlas U. S., U. S. Geol. Survey, illustration sheet, 



fig. 52. 

 1906. Exogyra costata B6se, Bol. Mexico Inst. Geol., No. 24, pp. 51-54, PL VI, fig. 3, PL VII, fig. 1, PL VIII, figs. 2-3. 

 PL IX, fig. 3. 



1906. Exogyra costata Veatch, Prof. Paper U. S. Geol. Survey No. 46, PL XI, fig. 2a-b. 



1907. Exogyra costata Weller, New Jersey Geol. Survey, Paleontology, vol. 4, text, pp. 456^58, PL XLVII, fig. 1. 



Description. — Shell of adult specimens thick and massive, becoming ponderous in some 

 overgrown specimens; subcircular to subovate in outline; dimensions of an average-sized 

 specimen, length 105 millimeters, height 115 millimeters, convexity 65 millimeters; the dimen- 

 sions of the largest specimen in the collections, length 162 millimeters, height 200 millimeters, 

 convexity 100 millimeters (PI. XVIII); valves unequal. Left or lower valve much larger 

 than right valve, convex, and attached in proximity to beak to an external object; the beak 

 usually more or less deformed by the scar of attachment; apical portion of shell spirally 

 coiled within the margin; hinge and other internal shell characters essentially the same as in 

 Exogyra ponderosa Roemer; outer surface of shell characterized by regularly arranged, promi- 

 nent, often rugged, radiating, entire or bifurcating costse, which in typical normal specimens 

 extend in curves conforming to the spiral twist of the shell from the beak to the margin; the 

 costse are separated by depressions which are usually narrower than the costse themselves; in 

 occasional nontropical specimens the costse are weakly developed and in large overgrown indi- 

 viduals the costse become faint or disappear entirely in the direction of the margin (PI. XVIII) ; 

 in cross section the costse vary in shape from semicircular to squarish; the costse vary in 

 maximum width on different adult individuals from 3 to 8 millimeters; many specimens are 

 further ornamented by concentric, imbricating growth lamellae, which vary greatly in promi- 

 nence from mere growth lines to broad, thin, projections, in some specimens extending outward 

 from the summits of the costse in spinelike folds (PI. XIX, fig. 1); in some specimens the sum- 

 mits of the costse are ornamented with slight nodular protuberances (PI. XIX, figs. 3 and 4) ; 

 in most specimens there is a more or less distinctly marked umbonal ridge extending from the 

 beak around to the posterior margin in a curve corresponding to the spiral twist of the shell; 

 along the umbonal ridge the costse bifurcate frequently, those in front and below the ridge 

 extending with a slight backward curve to the lower margin, and those above and to the rear 

 of the ridge extending with a strong, upward curve to the upper posterior margin. Upper or 

 right valve flatly spiral, roughly disk-shaped or operculiform, the outer surface varying from 

 slightly convex to slightly concave, the valve inclosed within and slightly depressed below the 

 projecting margin of the lower valve; hinge and other internal shell characters essentially the 

 same as in Exogyra ponderosa Roemer; beak depressed, not prominent; surface of shell orna- 

 mented with numerous concentric sharp-edged lamella?, separated by narrow, deep depressions; 

 radiating costse absent or but faintly developed on most specimens but on some specimens are 

 fairly prominent on upper posterior one-third of the shell surface (PI. XX, fig. 1). 



Remarks. — The exact locality from which the type specimen of this species was taken is 

 not known, but it is believed by Weller to have come from Mullica Hill, N. J., a Navesink marl 

 (Monmouth group) locality. In general this species occurs in beds stratigraphically higher 

 than those in which the species Exogyra ponderosa occurs, but there are apparent exceptions. 

 (See p. 44.) 



The specimens taken from the base of the zone of Exogyra costata have in general a coarser 

 surface ornamentation than those from higher horizons. (See p. 43.) 



Geologic occurrence. — In the eastern Gulf region the species is generally distributed through 

 the zone of Exogyra costata, which includes the Ripley formation (typical marine beds) of north- 

 ern Mississippi, approximately the upper one-half of the Selma chalk, where this formation is 

 most fully developed, and the upper one-half or two-thirds of the Riple\ r formation (typical 



