206 SYSTEMATIC PALEONTOLOGY 



their corresponding teeth; anterior margin cordate, flattened, distin- 

 guished f om the disk by an abrupt angular ridge ; posterior edge rounded, 

 very short; inferior edge nearly rectilinear, or contracted in the middle." 

 Say, 1822. 



" This is the type of the subgenus. In this species the beaks are more 

 nearly in the middle than in either of the others. [A limula Conrad and 

 A. incile Say]. The ligament does not occupy the whole of the cardinal 

 area, and the greater portion of it is in front of the beaks and strongly 

 transversely striated. The borders of the adductor scars are sometimes 

 marked by an elevated ridge as strong as in many Cucullseas." Dall, 1898. 



This large species is not as common as A. transversa, but is by no 

 means a rare form. It has been found in the Pleistocene from New 

 Jersey to Florida and in the Eecent ranges from Cape Cod to Yucatan. 



Width 63 mm.; height, 50 mm. 



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

 bor, St. Mary's County. 



Collections. — Maryland Geological Survey, Johns Hopkins University, 

 and U. S. National Museum. 



Subgenus SCAPHAKCA (Gray) Dall. 



Arca (Scaphaeca) teansveesa Say. 

 Plate LXIV, Figs. 7-10. 



Area transversa Say, 1822, Jour. Acad. Nat. Sci. Phila., vol. ii, 1st ser., p. 



269. 

 Arca transversa Conrad, 1832, Fos. Tert. Form., p. 15, pi. i, fig. 2. 

 Arca transversa Holmes, 1858, Post-Pi. Fos. S. C, pp. 21, 22, pi. iv, figs. 



5, 5a. 

 Arca (Scapharca) transversa Dall, 1889, Bull. U. S. Nat. Mus., No. 37, p. 40, 



pi. lvi, fig. 2. 

 Arca ( Hcapharca) transversa Dall, 1898, Trans. Wagner Free Inst. Sci., vol. 



iii, pt. iv, p. 645. 



Description. — " Shell transversely oblong, rhomboidal, with from 

 thirty-two to thirty-five ribs, placed at nearly the length of their own 

 diameters distant from each other; apices separated by a long narrow 

 space, and situate at the termination of the posterior third of the length 



