MARYLAND GEOLOGICAL SURVEY 387 



Description. — " Shell large, heavy, inflated, short, with small, high, 

 somewhat prosocoelons beaks, the two halves of the wide cardinal area 

 inclined to one another in the adnlt at an angle of about forty-five 

 degrees; left valve with about thirty strong, flattened subequal 

 radial ribs -^vith narrower interspaces; in the young the ribs 

 are" furnished Avith small transverse nodulations, which gradually 

 become obscure in the adult: the only transverse sculpture is of the 

 ordinary incremental lines; the ribs in the adult are flat-topped and 

 rarely show any tendency to mesial sulcation, and when present it 

 appears only on a few of the anterior ribs near the margin ; the anterior 

 end is obliquely rounded to the base, the posterior end a little produced 

 basally; the cardinal area is exceptionally wide, with a single impressed 

 line joining the beaks and six or seven concentric lozenges deflned by 

 sharp grooves; a deep groove also bounds the area; hinge line straight 

 with numerous small vertical teeth, becoming much larger distally and 

 tending to break up into granules at both ends of the series in the senile 

 shell. . . ." Dall, 1898. 



This shell seems more closely related to A. idonea than to any other. 



Length, 51 mm. ; height, 53 mm. ; diameter, 53 mm. 



Occurrence. — St. Mark's Formation. St. Mary's Eiver, Crisfield 

 v/ell at depth of 140 feet (U. S. National Museum). 



Collections. — Maryland Geological Survey, TJ. S. National Museum. 



Section SCAPHARCA ss. 



Arca (Scapharca) staminea Say. 

 Plate CV, Figs. 2, 3, 4, 5, 6. 



Area staminea Say, 1832, Amer. Couch., pL xxxvi, tig. 2. 



Arca elevata Conrad, 1840, Fossils of the Medial Tertiary, No. 1, 2q p. ol cover. 



Arca callipleura Conrad, 1840, Fossils of the Medial Tertiary, p. 54, pi. xxix, fig. 2. 



Arca triquelra Conrad, 1843, Proc. Acad. Nat. Sei. Phila., vol. i, p. 30.5. 



Arca triquetra Conrad, 184.5, Fossils of the Medial Tertiary, p. 59, pi. xxxi, He:. 2. 



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



Scapharca triquetra Conrad, 1863, Proc. Acad. Nat. Sci. Phila., vol. xiv, p. .580. 



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



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



Arca {Scapharca) callipleura Whitfield, 1894, Mon. xxiv, U. S. Geol. Survey, p. 43, 



pi. vi, figs. 8, 9. 

 Scapharca (Scapharca) staminea Dall, 1898, Trans. Wagner Free Inst. Sci., vol. iii, 



pt. iv, p. 642. 



