386 SYSTEMATIC PALEONTOLOGY 



limbo, rounded near the base; ribs sulcated by a strong median groove, 

 supplemented usually by a finer groove on either side of the median one ; 

 posterior side flattened, ribs there but slightly prominent. 



Length, 53 mm. ; height, 34 mm. ; diameter, 15 mm. 



Occurrence. — Calvert Formation. Chesapeake Beach, 3 miles south 

 of Chesapeake Beach, Plum Point, Truman's Wharf, White's Landing, 

 Church Hill, 3 miles west of Centerville, Eeed's, Wye Mills, near 

 Skipton. 



Collections. — Maryland Geological Survey, Johns Hopkins University, 

 U. S. National Museum. 



Arca (Scapharca) elnia n. sp. 

 Plate CIV, Figs. 4a, 4b. 



Description. — Shell large, moderately thick, but slightly elongated, 

 not inflated, with prominent prosocoelous beak; cardinal area wide, with 

 numerous irregular, zigzag, longitudinal grooves, bounded by a single 

 deep curved groove from the beak to the ends of the hinge line; hinge 

 line narrow; teeth small, obsolete medially, tending to become irregular 

 at both ends of the series; right valve with about thirty-one low ribs 

 hardly as wide on anterior dorsal slope as intervening spaces, broader 

 and more elevated on posterior dorsal slope; each rib mesially sulcated 

 by a groove with one or more subordinate grooves on either side ; growth 

 lines distinct; margin a continuous curve from anterior end of hinge 

 line to posterior end of base, there sharply curved ; posterior margin 

 oblique to hinge line; interior margin crenulated; dorsal and posterior 

 slopes meet in an angle that becomes rounded near the basal margin. 



This species seems to be intermediate between A. staminea and 

 A. suhrostrata, being perhaps more nearly related to the latter. 



Length, 60 mm.; height, 48 mm.; diameter, 22 mm. 



Occurrence. — Choptank Formation. Jones Wliarf, lower bed at 

 Governor Eun, 2 miles south of Governor Eun. 



Collection. — Maryland Geological Survey. 



Arca (Scapharca) clisea Dall. 

 Plate CV, Fig. 1. 



Scapharca (Anadara) clisea Dall, 1898, Trans. Wagner Free Inst. Sci.,vol. iii, pt. iv, 

 p. 657, pi. xxxiii, fig. 25. 



