SPECIES OF EXOGYRA FROM THE EASTERN GULF REGION AND THE 



CAROLINAS. 



By Lloyd William Stephenson. 



INTRODUCTION. 



In the preceding paper on "The Cretaceous deposits of the eastern Gulf region," the two 

 principal representatives of the genus Exogyra, namely, Exogyra ponderosa Roemer and Exogyra 

 costata Say, have been used to designate the two principal faunal zones recognizable in the 

 marine Upper Cretaceous deposits of the eastern Gulf region. On account of the prominence 

 thus given to the genus and the inadequacy of the information previously published concerning 

 its representatives in the region, the inclusion of the present paper in this report is deemed 

 appropriate. . 



The United States National Museum contains a large collection of Upper Cretaceous inver- 

 tebrate fossils obtained during the past 25 or 30 years from localities in the eastern Gulf region 

 and in the Carolinas. These have been collected chiefly by L. C. Johnson, T. W. Stanton, and 

 the writer. A number of other persons have made small contributions. 



Among these collections are many specimens of Exogyra from horizons between the base 

 and the top of the marine Cretaceous deposits. Indeed, here, as also throughout nearly the 

 whole of the marine Upper Cretaceous deposits of the Atlantic and Gulf Coastal Plain, repre- 

 sentatives of the genus are among the most abundant and generally distributed fossils. 



All the species and varieties of the genus now recognized hi the eastern Gulf region and 

 the Carolinas have in the past been referred by many authors to the one species Exogyra costata 

 Say, though some of them have been recognized as worthy of at least varietal rank and as having 

 different stratigraphic ranges. However, in Texas the relative stratigraphic positions of 

 Exogyra ponderosa Roemer and Exogyra costata Say have long been a matter of published record, 

 and in New Jersey a similar restriction in the ranges of these two species has been recognized by 

 Stuart Weller. 1 Weller also recognized that the two species were similarly restricted in range 

 in the South, basing his conclusions on collections from the Southern States, which he examined 

 in the National Museum. 



The purpose of this paper is to demonstrate that the representatives of the genus in these 

 regions are separable into at least three species, two of which present well-characterized varieties, 

 and to show that these forms are restricted in range in such a maimer as to give them distinct 

 value in correlation. The statements here given as to the range, distribution, and occurrence 

 of the species and varieties of Exogyra in the eastern Gulf region and the Carolinas are based 

 on a study of the collections in the National Museum and on personal field observations, except 

 as otherwise stated. 



DEFINITION OF THE GENUS. 



The genus Exogyra is a member of the family Ostreidse, superfamily Ostracea. It is distin- 

 guished from Ostrea and Gryphsea, the other principal genera of the family, chiefly by its spirally 

 curved beaks. The genus was first described by Say, 2 the type being Exogyra costata Say. 

 His description is as follows : 



Shell inequivalve, inequilateral; cicatrix one, large, deeply impressed, subcentral; inferior valve convex, 

 attached, umbo spiral, spire lateral, prominent, hinge with two parallel, transverse grooves; superior valve discoidal 

 operculiform, umbo not prominent, revolving spirally within the margin, hinge with a single groove on the edge. 



1 The Cretaceous paleontology of New Jersey: New Jersey Geol. Survey, Paleontology, vol. 4, text, 1907, pp. 456-460. 



2 Say, Thomas, Am. Jour. Sci., 1st ser., vol. 2, 1S20, p. 43. 



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