Maryland Geological Survey 563 



North and South Carolina. Ripley Formation. Exogyra costata zone, 

 Georgia; Eufaula, Alabama. Nacatoch Sand. Arkansas. Arkadelphia 

 Clay. Arkansas. 



Genus EXOGYRA Say 

 [Am. Jour. Sci., 1st ser., vol. ii, 1820, p. 43.] 



Type. — Exogyra costata Say. 



"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." — Say, 1820. 



This genus, by reason of its ponderous lower valve and considerable 

 abundance, is the most conspicuous element in the Upper Cretaceous 

 faunas of Maryland, particularly those of Prince George's County. It is 

 constantly separated from the other Ostreida by the twisted umbones and 

 ligament pit. In Maryland it runs larger and heavier than any other 

 member of the family and greatly exceeds them all in the discrepancy of 

 the valves. 



Douville, 1 in his " Observations sur les Ostreides : Origine et Classifica- 

 tion," has advanced the theory that the Exogyra were denizens of shal- 

 low waters and that the gyrate umbones and convexo-plano outline were 

 the result of resistance to the strong, rapid currents, while the less inequi- 

 valve and inequilateral Pycnodontw lay in the deeper waters out of the 

 influence of the strong current action. The strength of this argument is 

 somewhat vitiated by the fact that, in Maryland and more particularly in 

 Delaware, along the Chesapeake and Delaware Canal, the two genera are 

 commingled in great abundance. 



A. External surface radially costate or cancellate at least in the unibonal 



region. 



1. External surface radially costate, at least in the unbonal region. 



Exogyra costata 



2. External surface cancellate in the umbonal region. 



Exogyra costata var. cancellata 



B. External surface not radially costate nor cancellate. . . .Exogyra ponderosa 



Etymology: e£w, outside; -yvpos. a circle. 



1 Douville, 1910, Bull. Soc. G6ol. de France, 4e ser., tome x, pp. 634-645. 



