53 



AWHKICAN IsAST COAST ARCAS 53 



to incurvation of the posterior basal margin; it is a smaller shell with more posterior 

 beaks, and less roundly inflated. We have a large series of this species from many lo- 

 calities, and these differences characterize them all. The forms are easily diflerenliated, 

 so far as our ])resent knowledge goes, and therefore are better kept apart. In all the 

 pairs of A. slamiuca in the collection the right valve is distinctly smaller than and fits 

 into the other, while in A. disea the margins meet evenly." — Dall, 1898. 



Glenn thinks this shell is more closely related to A. idonca than to any other. 

 Some of the short, high forms of A. staminea approach A. disea. A left valve from Ev- 

 ergreen, Virginia, which resembles A. disea is related to A. staminea, but is well rounded 

 instead of angular. The specimen figured is related to idonea. 



Occurrence. — Chesapeake Miocene of Maryland, and at St. Mary's River and Cris- 

 field; of Nomini Cliffs, Virginia; and of Walton County, Florida. — Dall. St. Mary's 

 Miocene of St. Mary's River, Crisfield well at depth of 140 feet. — Glenn. Miocene of 

 North Carolina. — C. !■. Museum. 



Area aresta Dall 

 Plate XII, Figures 9, 10, 11 



Scapharca (Aniu/ara) aresla Dall, Wagner Free Inst. Sci., Trans., vol. 3, pt. 4, pp. 638, 655, pi. 33, 

 fig. 2, 189S. 



"Shell of moderate size and thickness, arcuate below, straight above, with small 

 but prominent prosocoelous beaks, left valve with twenty-seven square-topped, narrow, 

 entire radial ribs, separated by wider interspaces; the ribs on the middle of the shell are 

 somewhat narrower than the others; all are crossed by evenly spaced, moderately prom- 

 inent elevated lines, festooned in the interspaces, and forming small, square ripples on 

 the ribs; both valves similarly sculptured; cardinal area narrow, with elevated margins 

 behind, wider and short in front of the beaks; the portion in front of the beaks is longi- 

 tudinally striated, behind the 'beaks there are three or four concentric, lozenge-shaped 

 groovings; a single transverse groove usually passes between the beaks; hinge-line 

 straight; teeth in two nearly equal series, overlapping a little proximally, the teeth 

 rather crowded and nearly vertical; base of the valves arcuate, rounded into the anterior 

 end, posterior end a little produced; internal margins of the valves fluted. Lon. 41, 

 alt. 28, diam. 26 mm. 



"This very neat and distinct species appears to be the most common Ark in the 

 upper or Miocene bed at Alum Bluff." — Dall, 1898. 



Ribs twenty-four to twenty-nine, usually twenty-seven; beaks mesially sulcate. 

 The prominent characters of this species are the broad, rounded and festooned inter- 

 spaces, the narrow, square-topped ribs and the unusual form. The anterior and poster- 

 ior extremities are both nearer to the hinge-line than to a parallel line through the low- 

 est point of the base; the posterior portion is often attenuated, but in young specimens 

 this part is relatively broader and more evenly rounded; the greatest inflation is anterior 

 to the middle of the shell. 



