GEOLOGIC AND GEOGRAPHIC RANGE. 43 



characters described by him, many of the specimens have on the beaks of the larger valves 

 small regularly arranged costse extending from near the tip of the beaks backward over the sur- 

 face of the shell for one-half to three-quarters of an inch, and some of them have a series of more 

 or less strongly developed, sharp to round ridged costse, which extend backward from the t>eak 

 to a maximum distance of 5 inches, and which are strikingly irregular as regards their promi- 

 nence, shape, and distribution. That these are variations within the species is proved by the 

 fact that specimens showing all gradations from those entirely devoid of radiating costse to 

 those possessing the distinctly developed irregular costse may in places be obtained from the 

 same horizon. Although the two extremes are connected by forms showing all intermediate 

 gradations, the striking difference in appearance between the noncostate forms and those with 

 the well-developed irregular costse seems to justify the adoption of a distinguishing designation 

 for the latter, and the name Exogyra ponderosa var. erraticostata is therefore proposed. 



The original of Exogyra costata was described by Say 1 from the Cretaceous of New Jersey. 

 The exact locality from which the type was obtained is not known but is believed by Weller 2 to 

 have been Mulica Hill, a Navesink marl (Monmouth group) locality. 



The species is characterized by the presence on the larger valve of regularly arranged, 

 prominent, radiating, entire or bifurcating costse, which, except on large, overgrown individ- 

 uals, extend from the beak to the margin of the shell. These costse vary in their greatest 

 width on different individuals from 3 to 8 millimeters. In general, the costse are widest in 

 specimens occurring in the lower beds of the stratigraphic range of the species and grow nar- 

 rower in those in successively higher beds to the upper limit of the range, but there are excep- 

 tions to the rule. A distinct varietal form of the species, and one which may perhaps be 

 deserving of specific rank, is characterized by rather weak, regularly arranged, radiating costse, 

 interrupted by regularly distributed concentric depressions or undulations in such a manner as 

 to give to the surface of the shell a checkered or cancellated appearance. For this form the 

 name Exogyra costata var. cancellata is proposed. 



GEOLOGIC AND GEOGRAPHIC RANGE. 



EASTERN GULF REGION AND THE CAROLINAS. 



In Plate XII the known ranges in the Atlantic and Gulf Coastal Plain of the species and 

 varieties of Exogyra described in this paper are shown diagrammatically. 



The known range of Exogyra upatoiensis sp. nov. in the eastern Gulf region is very slight, 

 both stratigraphically and geographically, the species apparently being restricted to a stratum 

 a few feet thick near the base of the Eutaw formation in eastern Alabama and western Georgia, 

 the known linear extent of which does not exceed 8 miles. Specimens have been collected at 

 three localities. Outside the eastern Gulf region the species is known from but one locality. 

 In a well boring at Charleston, S. G, it was obtained from samples representing the depths 

 1,974 to 2,007 feet. 



Exogyra ponderosa Roemer makes its first appearance near the base of the Tombigbee sand 

 member of the Eutaw formation in the Chattahoochee region and ranges upward to about the 

 middle of the Selma chalk and its corresponding nonchalky marine equivalents to the northward 

 and eastward. In northern Mississippi these nonchalky equivalents consist chiefly of glau- 

 conitic, more or less calcareous sands, which constitute a part of the Tombigbee sand member 

 of the Eutaw formation. In eastern Alabama and Georgia they consist of marine sands, clays, 

 and marls, making up the lower one-third or one-half of the Ripley formation. The species 

 has been obtained from 64 authentic localities scattered throughout a belt extending from 

 Chattahoochee County, Ga., through Alabama and Mississippi to Henderson County, Tenn., a 

 distance of about 375 miles, but by detailed collecting this number could be multiplied many 

 times. 



i Say, Thomas, Am. Jour. Sci., 1st ser., vol. 2, 1820, p. 43. 



1 Weller, Stuart, The Cretaceous paleontology of New Jersey: New Jersey Geol. Survey, Paleontology, vol. 4, text, 1907, pp. 458-45S. 



