MARYLAND GEOLOGICAL SURVEY 195 



Description. — "Shell triangular, very convex, of a smooth appearance, 

 but with very minute, transverse wrinkles; lateral margins flattened, 

 cordate, with a rectilinear, sometimes concave profile, one margin rounded 

 at the tip, the other longer and less obtuse; umbo nearly central, promi- 

 nent." Say, 1822. 



This common and widespread form shows considerable variation. It 

 is frequently so numerous in the Maryland Pleistocene as to form almost 

 solid layers of shells. 



This species makes its first appearance in the Miocene of North Caro- 

 lina and Mississippi, is common in the Pliocene of the Carolinas and 

 Florida and ranges in the Pleistocene from Maine to Texas. In the 

 Eecent it is found from New Brunswick south to the Florida Strait and 

 westward as far as Texas. 



Width, 19 mm. ; height, 13 mm. 



Occurrence. — Talbot Formation. Wailes Bluff near Cornfield Har- 

 bor, St. Mary's County, Sparrows Point well, Baltimore County. 



Collections. — Maryland Geological Survey, Johns Hopkins University, 

 and IT. S. National Museum. 



Genus RANGIA Desmoulins. 



Eangia cuneata (Gray). 

 Plate LV, Figs. 5-8. 



Gnathodon cuneata (Gray) Sowerby, 1831, Gen. Shells, pi. xl. 

 Gnathodon cuneatus Holmes, 1858, Post-PL Fos. S. C., p. 41, pi. vii, fig. 10. 

 Gnathodon cuneata Dall, 1889, Bull. U. S. Nat. Mus., No. 37, p. 62. 

 Gnathodon cuneata Dall, 1894, Monograph of Gnathodon, Proc. U. S. Nat. 



Mus., vol. xvii, pp. 97, 98, pi. vii, figs. 1 and 10. 

 Rangia cuneata Dall, 1898, Trans. Wagner Free Inst. Sci., vol. iii, pt. iv, p. 



904. 



Description. — Shell trigonal; umbones prominent; anterior shorter 

 than the posterior end; hinge comprising a bifid triangular cardinal tooth 

 in the left valve on which fit two lamellar, divergent teeth of the right 

 valve ; an anterior lateral tooth in the left valve, uncinate in form, fitting 



