EXOGYEA COSTATA CANCELLATA. 53 



U. S. Geol. Survey coll. 4143. — Northeast Cape Fear River, Hilton Park, Wilmington, N. C. Top of Peedee sand. 



U. S. Geol. Survey coll. 4147. — Burches Ferry, Peedee River, right bank, 87J miles above Georgetown, in 

 Florence County, S. C. (15 miles below Seaboard Air Line Ry. bridge). Peedee sand. 



U. S. Geol. Survey coll. 4159. — Davis Landing, Peedee River, right bank, 71 miles above Georgetown, in 

 Florence County, S. C. Peedee sand. 



EXOGYRA COSTATA Var. CANCELLATA Var. 110V. 



Plate XX, figures 2, 3. 4; Plate XXI. figures 1, 2. 



Description. — Shell of adult moderately thick; subcircular to subovate in outline: dimen- 

 sions of an average specimen, length 92 millimeters, height 89 millimeters, convexity 41 milli- 

 meters; dimensions of a rather large specimen, length 117 millimeters, height 123 millimeters, 

 convexity 58 millimeters (PI. XXI, fig. 2) . Left or lower valve much larger than right valve 

 and strongly convex; attached in proximity of beak to an external object, the beak usually 

 somewhat deformed by the scar of attachment; general form, hinge characters, and other 

 internal shell characters essentially the same as in Exogyra costata Say; surface of shell orna- 

 mented with more or less distinct, low, bifurcating, nodular costa?, the nodes produced by 

 concentric depressions regularly arranged in such a manner as to give to the surface of the shell 

 a checkered or cancellated appearance (PI. XX, figs. 2 and 3) ; the nodes on the costa? are 

 in some specimens more prominently connected concentrically than in the direction of the 

 radiating costa?, thus producing distinct concentric ridges (PI. XX, fig. 4); in nontypical 

 specimens the costa? are weakly developed and there is a corresponding strong development 

 of concentric growth lamella?; in adult specimens the costse, apparently 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 

 (PI. XXI, fig. 2) ; 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 (PI. XXI, fig. 2); along the posterior 

 margin of this depression, which perhaps corresponds to the umbonal ridge in Exogyra costata 

 Say, the radiating costa? 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 posterior margin of the shell. Upper or right 

 valve operculiform, roughly ovate hi outline and inclosed within the projecting margin of the 

 lower valve (PI. XXI, fig. 1); usually distinctly concave on outer surface and convex on inner 

 surface; 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 lamella?, separated by deep narrow depressions, 

 the lamella? behig more prominent toward the outer margin of the shell, the inner, strongly 

 concave portion of the surface behig nearly smooth (PI. XXI, fig. 1); costa? 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 ornamentation, always recognizable, which cer- 

 tainly 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. 



Morton 1 has described an oyster from New Jersey which he called Ostrea torosa, the orna- 

 mentation of which, as figured, suggests that it may have been an imperfect or distorted specimen 

 of this variety. The specimen was later referred by Gabb - to Exogyra costata. Weller, in his 

 monograph on the Cretaceous paleontology of New Jersey, makes no mention whatever of the 

 specimen, and probably it was not preserved and was not available for examination when his 

 monograph was written. 



1 Morton, S. G., Synopsis of the organic remains of the ferruginous sand formation of the United States: Am. Jour. Sei., 1st ser.. vol. 24, 1S33, p. 

 130, PI. X, fig. 1; Synopsis of the organic remains of the Cretaceous group, 1S34, p. 52, PI. X, fig. 1. 



2 Gabb, W. M., Synopsis of the Mollusca of the Cretaceous formation, 1861, p. 122. 



