46 SPECIES OE EXOGYEA EEOM THE EASTERN GULF REGION AND THE CAROLINAS. 



SYSTEMATIC DESCRIPTIONS. 



Genus EXOGYRA Say. 



EXOGYRA UPATOIENSIS Sp. nOV. 



Plate XIII, figures 1, 2, 3, 4. 



Description. — Shell small and thin as compared with other members of the genus in the 

 region; subcircular to subovate in outline; dimensions of a medium-sized individual, length 

 33 millimeters, height 35 millimeters, convexity 18 millimeters (PL XIII, fig. 1); dimensions 

 of a large individual, length 52 millimeters, height 55 millimeters, convexity 43 millimeters 

 (PI. XIII, fig. 4). Left or lower valve much larger than right valve, strongly convex; 

 attached in proximity of beak to an external object, the scar of attachment generally 

 large and causing much distortion of the shell; the object of attachment is frequently 

 the shell of another mollusk, the sculpture of which is distinctly impressed on some speci- 

 mens of Exogyra (PL XIII, fig. 1); apical portion spirally coiled within the marginal out- 

 line of the shell, the coils including 1 to 2 volutions; hinge with ligamental groove broad, 

 deeply impressed, paralleled above by a narrow, shallow groove, both grooves curved to con- 

 form to the spiral twist to the shell; posterior to the larger groove a broad shallow, striated 

 depression; adductor scar distinct; surface ornamented with numerous fine, narrow, fre- 

 quently bifurcating costse, characterized by their irregularity as regards shape, size, and 

 prominence (PL XIII, figs. 1 and 4) ; an umbonal ridge more or less clearly defined extends 

 from the beak in a curve conforming to the spiral twist of the shell to the lower posterior 

 margin — but this ridge in some specimens is interrupted in proximity to the beak by the scar 

 of attachment; along the umbonal ridge the costae bifurcate more numerously than elsewhere 

 (PL XIII, fig. 1); toward the margin of the larger individuals the costse become more irregular 

 and weaker, in some specimens disappearing entirely; concentric growth lines are present in 

 smaller individuals, and toward the margin of larger individuals these develop into prominent 

 imbricating lamella? (PL XIII, fig. 4). Upper or right valve, flat or slightly concave, opercu- 

 liform, subcircular, or subovate in outline, with nearly flat, spiral twist, the beak being well 

 within the margin ; beak depressei I, not prominent : this valve inclosed within and slightly depressed 

 below the projecting margin of the lower valve; hinge with broad deeply impressed ligamental 

 groove, curved to conform to the spiral twist of the shell, the upper margin of the groove 

 finely crenulated; posterior to the groove a striated protuberance occupies a position in appo- 

 sition to the similarly striated depression on the left valve; surface marked by fine concentric 

 growth lines which toward the margin are produced into thin projecting lamellae, separated by 

 deep, narrow depressions. 



Geologic occurrence. — Known oidy from beds near the base of the Eutaw formation in the 

 Chattahoochee region (Alabama and Georgia), and from a deep well at Charleston, S. C. 



Localities. ^The species has been obtained from four localities as follows: 



U. S. Geol. Survey colls. 5373. 5377. — Bluff on Upatoi Creek below bridge of ('olumbus-Cusseta road, 7 miles 

 southeast of Columbus, in Chattahoochee County, Ga. Near base of Eutaw formation. 



U. S. Geol. Survey colls. 847, 5384, 6408. — Broken Arrow Bend, t Chattahoochee River, 10J miles below Columbus, 

 Ga. Near base of Eutaw formation. 



U. S. Geol. Survey coll. 53S5. — Chattahoochee River, one-half mile below Broken Arrow Bend. Near base of 

 Eutaw formation. 



U. S. Geol. Survey coll. 7778. — Deep well of the Charleston Consolidated Railway & Lighting Co., Charleston, 

 S. C, from a sample representing the depths 1,974 to J. 007 feet. 



Exogyra poxderosa Roemer. 

 Plate XIII, figure- 5, ». 7; Plate XIV; Plate XV, figures 1, 2, 3. 



1849. Exogyra ponderosa Roemer, Texas, p. 395. 



1852. Exogyra ponderosa Roemer, Kreidebildungen von Texas, pp. 71-72. Taf. 9, figs. 2 a-b. 



1853. Exogyra ponderosa Shumard, Marcy"s Exploration of the Red River. La., pp. 204-205. 



1857. Exogyra costata (var. ) Conrad, U. S. and Mex. Boundary Survey, vol. 1, pt. 2, p. 154, PI. VIII, fig. 3, PI. IX, 

 fig. 1. 



