42 SPECIES OF EXOGYEA FROM THE EASTERN GULF REGION AND THE CAROLINAS. 



The several generic characters described by Say may be seen by referring to the illustrations 

 in this paper as f ollows : 



Relative sizes of lower and upper valves. — Lower valves, Plate XIII, figure 1, and Plate 

 XV, figure 1. Upper valves, respectively, of same individuals, Plate XIII, figure 2, and 

 Plate XV, figure 2. 



Character and position of cicatrix (adductor scar). — Lower valve, Plate XIX, figure 2, 

 Upper valve, Plate XV, figure 3. 



Characters of left (lower) valve. — Convexity, Plate XIII, figure 7, and Plate XVI, figure 4. 

 Examples of scar of attachment, Plate XIII, figures 1, 3, 4, 7. Position and spiral character of 

 umbo (beak), Plate XIII, figures 5, 6, 7. Character of hinge, Plate XIX, figure 2. 



Characters of right (upper) valve. — Shape, Plate XV, figures 2, 3. Position and spiral 

 character of umbo (beak), Plate XV, figure 2. Character of hinge, Plate XV, figure 3. 



GEOLOGIC RANGE OF THE GENUS. 



The genus Exogyra ranges in time from the Upper Jurassic to the close of the Cretaceous 

 period. In the eastern Gulf region, however, the genus is known only in the Upper Cretaceous 

 beds, its range here being from the base of the Eutaw formation upward to the top of the 

 Cretaceous. (See fig. 1 , p. 23.) The formations embraced in this range are the Eutaw formation 

 (including the Tombigbee sand member), the overlying Selma chalk, and the nonchalky marine 

 equivalents of the Selma chalk. The latter, in northern Mississippi, comprises a part of the 

 Tombigbee sand member of the Eutaw formation and all of the Ripley formation; in eastern 

 Alabama and Georgia it comprises the Ripley formation. 



In the Carolinas the range is somewhat more restricted than in the eastern Gulf region, the 

 genus first making its appearance in beds toward the upper part of the Black Creek formation, 

 which correspond approximately to the basal beds of the Selma chalk in its type region, and 

 ranging upward through the remainder of the Black Creek strata and through all the overlying 

 Peedee sand or to the top of the Cretaceous. 



GENERAL DESCRIPTION OF THE SPECIES AND VARIETIES. 



In the region under consideration three species of Exogyra have been recognized, two of 

 which present variations of sufficient constancy to rank as varieties. 



Exogyra upatoiensis sp. nov. has been found at one horizon only, and its known geographic 

 range along the strike is only about S miles. Its horizon is within a few feet of the base of the 

 Eutaw formation in eastern Alabama and western Georgia, and is the lowest horizon at which 

 the genus has been found east of Mississippi River. 



In shape and general proportions the species is similar to the other members of the genus 

 from this region, but it is much smaller, the largest specimens in the collections not exceeding 

 2} inches in length. The lower or larger (left) valve is marked on its outer surface by numerous 

 fine, closely set, bifurcating, radiating costae, which present great irregularity as regards shape, 

 size, and prominence, this peculiar ornamentation constituting the chief distinguishing character 

 of the species. 



The original of Exogyra ponderosa, described by Roemer, 1 is from the vicinity of New 

 Braunfels, Tex. The species as described is characterized by the ponderous size of the adult 

 specimens, by the faint development of the radiating costse, and by the rather strong develop- 

 ment of concentric imbricating lamellae. However, among a number of specimens collected by 

 Stanton from the type locality are some with fairly well developed irregular costa? corresponding 

 closely to those on the varietal form of the species from the eastern Gulf region {Exogyra ponderosa 

 var. erraticostata) . 



The specimens from the eastern Gulf region and the Carolinas referred to Exogyra ponderosa 

 agree in all their essential characters with Roemer's species. However, in addition to the 



' Roemer, Ferdinand, Texas, Bonn, 1S49, pp. 395-396; Die Kreidebildungen von Texas, Bonn, 1S52, pp. 71-72. 



