36 



CKETACEOUS DEPOSITS OF THE EASTERN GULF REGION. 



Mollusca — Continued . 



D . ? subfilosa Conrad . a . 

 Acteon linteus Conrad, a. 

 Ringicula pulchella Shumard . 

 Bullopsis cretacea Conrad, a. 

 Bulla macrostoma Gabb. a. 

 B. mortoni Forbes, a. 

 Cylichna recta Gabb . a . 

 Nautilus dekayi Morton, n. 

 Baculites columna Morton, a 



Mollusca — Continued . 



Baculites labyrinthicus Morton. 

 B. tippaensis Conrad, a. 

 B . carinatus Morton . a. 

 Scaphites conradi (Morton), a. 

 Sphenodiscus lenticularis (Owen). 

 S. pleurisepta (Conrad), a. 

 Hamites sp. no v. a. 

 Turrilites alternatus Tuomey. a. 

 Belenmitella americana (Morton). 



Of the 187 species listed above, 127 are restricted to the Exogyra costata zone; 57, ques- 

 tionably 59, range below it. One species which occurs at a lower horizon is questionably present 

 in this zone. 



SUB-ZONES. 



OCCURRENCE OF THE FOSSILS. 



In the eastern Gulf region the zone of Exogyra costata is relatively prolific in species as 

 compared with the zone of Exogyra ponderosa, the former containing about 85 per cent more 

 than the latter. Although no exact figures have been prepared, much the greater number of 

 species have come from the upper hah of the zone. The most prolific fossil localities are found 

 in the Eipley formation in Tippah and Union counties, Miss., and in the bluffs of Chattahoochee. 

 River from Eufaula, Ala., downstream to the mouth of Pataula Creek, Ga. That part of the 

 zone represented by the Selma chalk (see Pis. IX in pocket and X, p. 20) in Alabama and 

 Mississippi contains fewer species than the part represented by the Ripley type of materials. 

 With the exception of a horizon near the top of the chalk formation, to be described below, the 

 Selma chalk fossils (not including the Forammifera) belong chiefly to the Ostreidse and Anomiidse 

 families. This relative paucity of species in the chalk formation may be clue in part to unfavor- 

 able life conditions in the area where chalk was being deposited, or to unfavorable conditions 

 of preservation, or to both. 



LIOPISTHA PROTEXTA STJBZONE. 



Toward the close of the deposition of the chalk formation the conditions for the existence 

 of life and for the preservation of life remains were favorable, as is shown by the presence of a 

 considerable fauna in the upper 40 or 50 feet of the terrane, here designated the Liopistha 

 protexta subzone. This subzone is traceable along the top of the Selma chalk in western Alabama 

 and east-central Mississippi, being coincident in the former State with the thin tongue of chalk 

 rocks which runs eastward above the sands of the Ripley formation hi Marengo and Wilcox 

 counties, and hi the latter State with the similar tongue of chalk running north through Oktib- 

 beha, Clay, and Chickasaw counties. (See red line 3, Pis. IX and X.) Excepting the Ostreidse, 

 Pectinidse, Spondylidse, Limidse, and Anomiidse, the shells of which are usually preserved, the 

 fossils present in the Liopistha protexta subzone in the Selma chalk are preserved chiefly hi the 

 form of casts, the materials of which are, as a rule, more or less phospha tic. Of the 40 or 50 

 species obtained the forms listed below appear to be restricted, within the area of the Selma chalk, 

 to the Liopistha protexta subzone; some of them, however, as explained below, are known to 

 have a somewhat greater stratigraphic range in the nonchalky equivalents of the chalk elsewhere 

 in the eastern Gulf region and northward in the Atlantic coast region. Because of its wide 

 geographic distribution in the uppermost beds of the Selma chalk the species Liopistha protexta 

 Conrad has, for convenience, been chosen to designate the subzone. 



