EXOGYEA PONDEEOSA. 47 



1869. Ostrea torosa Coquand (in part). Monographie du genre Ostrea, terrain cr<jtac6, Paris, p. 38, PI. IX, figs. 



1, 2, and 3. 



1870. Exogyra ponderosa Credner, Zeitschr. Deutsch. geol. Gesell., vol. 22, p. 229. 



1875. Exogyra -ponderosa White, Rept. Geog. Surveys W. 100th Mer., vol. 4, pt. 1, p. 172, PI. XIV, figs. 1 a-c. 



1884. Exogyra ponderosa White, Fourth Ann. Rept. U. S. Geol. Survey, p. 306, PI. L, figs. 1-2. 



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



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

 1893. Exogyra ponderosa Stanton, Bull. U. S. Geol. Survey No. 106, pp. 65-66, PI. VII, figs. 1 and 2. 



1901. Exogyra ponderosa Hill, Twenty-first Ann. Rept. U. S. Geol. Survey, pt. 7, PI. XLV, fig. 1. 



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



sheet, fig. 46. 



1906. Exogyra ponderosa Veatch, Prof. Paper U. S. Geol. Survey No. 46, PI. IX. 



1907. Exogyra ponderosa Weller, New Jersey Geol. Survey, Paleontology, vol. 4, text, pp. 458^60, PI. XLVII, 



fig. 2. 



Description. — Shell of adult very thick and ponderous, in outline subcircular to extended 

 subovate; dimensions of an adult individual, length 111 millimeters, height 177 millimeters, 

 convexity 94 millimeters (PL XIV) ; dimensions of a medium-sized specimen, length 97 

 millimeters, height 112 millimeters, convexity 60 millimeters (PI. XV, fig. 1). Left or 

 lower valve much larger than right valve, strongly convex; attached in proximity to beak 

 to external object, this part of the shell often very much deformed by scar of attachment; 

 apical portion of shell spirally coiled within the marginal outline of the shell; hinge with 

 ligamental groove broad, deeply impressed, paralleled on the upper side by a rather faintly 

 developed, narrow shallow groove, both grooves curved to conform to spiral twist of shell; 

 posterior to the larger groove a broad, shallow, pitted or striated depression; surface of 

 shell marked by thin, rather prominent, concentric, imbricating growth lamella?, with inter- 

 mediate fine growth lines; costa? either entirely absent or small, regularly arranged costa? 

 present in proximity to beak and extending back from beak one-half to three-fourths inch 

 (PI. XIII, figs. 5-7), or, in addition to the preceding, very faint irregular costa? extending back 

 to varying distances away from beak; a more or less clearly defined umbonal ridge extends 

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

 margin, usually, however, becoming rounder and less clearly recognizable toward the margin 

 (PI. XV, fig. 1). Upper or right valve flat or slightly concave, operculiform, subcircular 

 or subovate in outline, with a nearly flat, spiral twist, the beak being well within the 

 margin; beak depressed, 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 shell, the upper margin of the groove finely 

 crenulated (PI. XV, fig. 3) ; posterior to the groove a striated protuberance occupies a position 

 in apposition to the s imil arly striated depression on the left valve; in proximity to the beak 

 the surface is marked by numerous fine, concentric growth lines, which away from the beak 

 toward the margins are produced into thin projecting lamella?, separated by deep, narrow 

 depressions. 



Geologic occurrence. — In the Chattahoochee region (Alabama-Georgia) the species makes 

 its first appearance near the base of the Tombigbee sand member of the Eutaw formation. 

 It is common in the upper one-fourth to one-half of the Tombigbee sand hi western Georgia, 

 Alabama, and as far north in Mississippi as Monroe or Itawamba County, and hi that part of 

 the Tombigbee sand which in northern Mississippi represents the time equivalent of the basal 

 part of the Selma chalk. From the Tombigbee sand it ranges upward to about the middle of 

 the Selma chalk where the latter is most fully developed in western Alabama and east-central 

 Mississippi; and is present in the corresponding nonchalky marine equivalents of the lower 

 hah of the Selma chalk in eastern Alabama and in Georgia, these equivalents constituting the 

 lower one-third or one-half of the Ripley formation of this region. The upper limit of the range 

 of the species is represented in Plates IX and X by red line 2. 



In North Carolina the species occurs in the upper marine invertebrate-bearing beds of the 

 Black Creek formation. 



