Maryland Geological Survey 567 



a manner as to give to the surface of the shell a checkered or cancellated 

 appearance ; the nodes on the costaa are in some specimens more promi- 

 nently connected concentrically than in the direction of the radiating 

 costae, thus producing distinct concentric ridges, in non-typical specimens 

 the costae are weakly developed and there is a corresponding strong devel- 

 opment of concentric growth lamellae ; in adult specimens the costae, appa- 

 rently without exception, become faint and disappear in the direction of 

 the margin, there being an area bordering the margin, varying in width, 

 on which concentric imbricating lamella? form the only ornamentation ; 

 extending from the beak to the posterior margin in a curve corresponding 

 to the spiral twist of the shell there is a more or less distinctly defined, 

 shallow, depressed area which broadens gradually in the direction of the 

 margin ; along the posterior margin of this depression, which perhaps 

 corresponds to the umbonal ridge in Exogyra costata Say, the radiating 

 costae repeatedly bifurcate, those in front of this margin extending down- 

 ward in the direction of the lower margin of the shell and those behind 

 the margin extending upward in a rather sharp curve to the upper pos- 

 terior margin of the shell. Upper or right valve operculiform, roughly 

 ovate in outline and inclosed within the projecting margin of the lower 

 valve ; usually distinctly concave on outer surface and convex on inner sur- 

 face; hinge and other internal characters essentially the same as in 

 Exogyra costata Say; beak depressed, not prominent, with nearly flat 

 spiral twist or coil ; surface ornamented with numerous concentrically 

 arranged sharp-edged lamellae, separated by deep narrow depressions, the 

 lamellae being more prominent toward the outer margin of the shell, the 

 inner, strongly concave portion of the surface being nearly smooth ; 

 costae either absent or but very faintly developed toward postero-dorsal 

 margin. 



Remarks. — This variety has not previously been differentiated from the 

 typical form of the species. However, it possesses a distinctive ornamenta- 

 tion, always recognizable, which justifies its recognition as a variety ; there 

 is even a suggestion that the form developed parallel to rather than from 

 Exogyra costata Say, in which case it should, perhaps, be given specific 

 and not varietal rank." — Stephenson, 1914. 



