388 SYSTEMATIC PALEONTOLOGY 



Description. — " Shell thick prominently convex ; with about twenty- 

 eight ribs which are rounded and narrower than the intervening spaces, 

 excepting on -the anterior side, where they are broader, and simply 

 wrinkled, those of the anterior part of the disk have one or two longi- 

 tudinal impressed lines ; they are crossed by numerous transverse, elevated 

 lines, which are hardly more distant from each other than their own 

 width ; intervening spaces wrinkled : beaks distant, curved a little back- 

 ward, and the tip a little behind the hinge margin : area flattened, a 

 little curved, rather spacious, with obvious impressed, oblique lines : 

 hinge margin rectilinear, with small, numerous teeth : posterior margin 

 regularly arcuated : base subrectilinear, very deeply crenated : anterior 

 margin oblique, rectilinear: anterior side abruptly compressed." Say, 

 1832. 



Shell very elevated and ventricose ; umbonal and posterior slopes form- 

 ing almost a right angle, near which the ribs are striated instead of 

 granulated ; basal margin regularly curved or in the more elongated speci- 

 mens slightly incurved posteriorly. 



A careful comparison of what are doubtless the type specimens of 

 .4. caUipleura. shows that it is but a short, elevated, thickened and well 

 sculptured form of A. staminea. 



Length, 44 mm.; height, 38 mm.; diameter, 21 mm. 



Occurrence. — Choptank Formation. Governor Eun, 2 miles south 

 of Governor Eun, Flag Pond, Jones Wliarf, Cuckold Creek, Turner, 

 Dover Bridge, Peach Blossom Creek, Greensboro. 



Collections. — Maryland Geological Survey, Johns Hopkins T^niversity, 

 U. S. National Museum, Philadelphia Academy of Natural Sciences. 



Arca (Scapharca) aeata Say. 

 Plate CV, Figs. 7a, 7b. 



Area arata Say, 1824, Jour. Acad. Nat. Sci. PliiLa., voL iv, Istser., p. 137, pL x, fig. 1. 

 Arca arata Conrad, 184.5, Fossils of the Medial Tertiary, p. 58, pi. xxx, flg. 6. 

 Scapharca arata Conrad, 1863, Proc. Acad. Nat. Sci. Phila., vol. xiv, p. 579. 

 Scapharca arata Meek, 1864, Miocene Check List, Smith. Misc. Coll. (183), p. 6. 

 Scapharca {Scapharca) arata Dall, 1898, Trans. Wagner Free Inst. Sci., vol. iii, 

 pt. iv, p. 643. 



Description. — " Shell transversely oblong, subrhomboidal, with about 



twenty-six longitudinal ribs; basal edge nearly parallel to the hinge 



margin, which latter terminates anteriorly in an angle. 



