45 



Amkkican East Coast Arcas 45 



extremity of hinge angulated; basal margin straight or slightly contracted in the middle; 

 posterior margin oblique, emarginate above, extremity rounded; cardinal area narrow, 

 with two or three angulated grooves; series of teeth slightly arched towards the extrem- 

 ities; margin densely crenate. 



"Loc. Wibnington, N. C. 



"This is probably identical with a recent species of the southern coast, abundant in 

 the Post Pliocene marl of the Potomac river, St. Mary's Co., Md. The Miocene shell is 

 thicker than the more recent specimens." — Conrad, 1845. 



"This shell should not, in my opinion, be united with A. plicatura, as has been done 

 by Heilprin. When properly discriminated it is a smaller and more rhombical shell, 

 with lower and more anterior beaks, and more produced and pointed posterior end; the 

 base and hinge-line are nearly parallel, and the latter is narrower in specunens of the 

 same size than in -••:/ . plicatura. Both have about thirty-five ribs, but in A. improcera 

 these are plain, while in A. plicatura the anterior ribs are prettily nodulous. 



"Area buccula Conrad (Fos. Med. Tert., p. 60, pi. 31, fig. 4) appears to be a short, 

 heavy, stunted, and abnonnallj' thickened variety of this species, such as might be pro- 

 duced by an unfavorable environment. It is confined to the Upper Miocene marls of 

 Duplin County, North Carolina." — Dall. 



A. improcera, A. buccula, A. subsijmata, A. transversa and the forms placed by Dall 

 under A. plicatura are closely related, are difficult to separate satisfactorily and have 

 been arranged differently by different authors. The synonymy given here for improcera 

 and plicatura follows Dall. In general the specimens fall into groups corresponding with 

 certain species. Those with a small, thick shell, rather low posteriorly, oblong shape and 

 long hinge may be placed in A. improcera. There are short, thick shells from Duplin 

 County, North CaroHna, which Conrad called A. buccula, (Plate X. Figures 17, 18). A. 

 /rd?i5wr5a is thinner than A. improcera and higher posteriorly. The larger, irregular, 

 rounded shells may all be placed nnd^r plicatura for convenience. They vary much and 

 grade into both improcera and transversa. The distinctions Dall has made in the sculp- 

 ture oi plicatura and improcera will not hold true for all specimens. Some have the dis- 

 crepant sculpture and the nodulation of plicatura and the form of improcera. A. stebsin- 

 uata should perhaps be included in plicatura. Specimens from the Croatan beds show a 

 variation from the nearly straight base of subsinuata to the rounded base oi plicatura. 



Dimensions. — Lon. + 10,-18; alt. +3,-15; diam. 16 imn. 



Ocairrcnce. — Upper Miocene of Warwick, Virginia; of Duplin County and Wilming- 

 ton, North Carolina; of Timminsville and Darlington, South Carolina; Pliocene of the 

 Caloosahatchie River and Sliell Creek, Florida. — Dall. Miocene of Magnolia, North 

 Carolina, and Darlington Church House, South CaroUna. — C. U. Museum. 



Area plicatura Conrad 

 Plate X, Figures 19, 20 



Area plicatura Conrad, Fos. Med. Tert., p. 6i, pi. 32, fig. 4, 1S45; Heilprin, Acad. Nat. Sci. Pliila., 

 Proc. for 18S1, p. 451, 18S2 (ex parte). 



