34 Paleaontographica Americana 34 



The original description applies only to valves about the size of the type. The ma- 

 jority of the specimens are larger and with age they thicken and become much inflated, 

 the posterior part is more attenuate than in the smaller specimens and the ends of the 

 ribs often have a medial groove; the cardinal area has more grooves, some of which 

 usually extend in front of the beaks. There are twenty-nine to thirty-three ribs and the 

 cardinal margin is elevated behind and also slightly elevated in front near the beaks as in 

 A. hypomela. A. latidcntata is distinguished from the young of A. hypomela by its semi- 

 circular posterior margin. 



Dimensions. — Lon. +7,-18; alt. +2. 5, -12. 5; diam. 14 mm. A large valve is 28 mm. 

 long. 



Ocairrcncc. — Oligocene of Ballast Point, Tampa Bay, of the lower bed at Alum Bluff, 

 and in the Chipola marls of Florida, and probably from the Oak Grove sands in western 

 Florida. — Dall. Oligocene of Bailey's Ferry, Florida. — C. U. Museum. 



Area acompsa Dall 

 Plate VII, Figure 21 



Scapharca (Scapkarca) acotnpsa Dall, Wagner Free Inst. Sci., Trans., vol. 3, pt. 4, p. 648, pi. 33, fig. 

 15, 1898. 



"Shell rectangular, elongate, rather compressed, with low, prosocoelous beaks, situ- 

 ated at about the anterior fourth of the whole length ; right valve with about thirty-six 

 flattened radial ribs, with much narrower interspaces; the anterior (twenty-two) ribs are 

 mesially divided by a sharp groove and feebly rippled above; the posterior ribs are 

 flat, smooth, and increase in width backward; the anterior end of the shell is evenly 

 rounded, the base straight and parallel with the hinge-line, the posterior end wider, a 

 little produced below and with a conspicuous angle above ; cardinal area long, very nar- 

 row, with one or two grooves, and bordered behind with an elevated margin; hinge-line 

 straight, long, with numerous small, unintenoipted teeth very short mesially, longer and 

 somewhat more oblique distally ; inner margin of the valves fluted, shell thin and deli- 

 cate. Lon. 20, alt. 10.5, semi-diam. 4.5 mm. 



"Only two right valves of this little species have been examined. It resembles the 

 young of A. hypomela but is immediately distinguishable by its more compressed and 

 rectangular form and smooth, flat posterior ribs." — Dall, 1898. 



Occurrence. — Oligocene of the Chipola River, Florida, marl. — Dall. 



Area hypomela Dall 



Plate VII, Figures 22, 23, 24, 25 



Scapharca (.'Scapkarca) hypomela Dall, Waguer Free Inst. Sci., Trans., vol. 3, pt. 4, pp. 637, 6471 



648, pi. 33. fig- I. 1898. 

 Area (.Scapharca) hypomela Maury, Bull. .\m. Pal., vol. 3, p. 372, 1902. 



"Shell of moderate size, long, inflated, with rather low, mesially compressed, proso- 

 gyrate beaks; left valve with about forty-three deeply channelled, flat-topped ribs with 

 fine, regular, concentric beading, except on the posterior slope, where the ribs are lower. 



