BIVALVIA. 25 



Shell orbicular, subpellucid, inequivalved or somewhat plano-convex, equilateral, 

 externally covered with fine longitudinal rays, visible only near the margin, and beau- 

 tifully sculptured with diverging or divaricating striae, auricles unequal. 



Diameter, 2 inches. 



Locality. Cor. Crag, Ramsholt, Sudbourn, and Gedgrave. 



This beautiful species is exceedingly abundant at the latter locality, where, how- 

 ever, the valves are always separated. At Ramsholt, they are somewhat less abun- 

 dant, and the valves are there found in their natural position. 



From the figure and description given by M. Nyst, above referred to, there is little 

 doubt, that the English Crag shell is the same as the one found in the Campinian 

 beds of Belgium. Our shell is very thin, but strong, with irregularly-marked lines of 

 increase ; the right valve always more or less flattened, while the opposite one is 

 tumid or convex. Auricles very unequal and rayed, or costated, with a moderate 

 sized opening beneath the anterior one of the right valve, this is almost obliterated in 

 the adult shell, a rather large triangular cartilaginous area overhanging or projecting 

 into the interior. The impression formed by the edge of the mantle parallel with the 

 margin, and extending to about two thirds the length of the shell, that by the 

 adductor, less distinctly defined. The contour of the shell is nearly circular, a little 

 interrupted with a pair of rather high shoulders. A dark line diverges from the umbo, 

 within which the shell is of a lighter colour, like that in P. corneus, being, however, 

 perfectly distinct from that species. The beautifully curved or divaricating striae 

 (from the absence of longitudinal or radiating ribs), are visible upon the smooth sur- 

 face of the shell with the naked eye. An American fossil, Peden Virginianus, 

 Conrad. 'Foss. of the Med. Tert. of the United States,' p. 46, pi. 21, fig. 10, 1838, 

 appears to resemble our shell in many characters, but from the representation it differs 

 in being longer than high, and has the ears more equal and larger. 



3. Pecten siMiLis, LasTcey. Tab. V, fig. 4, a — c. 



Pecten SIMILIS. Laskey. Mem. Wern. Soc, vol. i, p. 387, pi. 8, fig. 8, 1811. 



— — Brown. lUust. Concli. Gr. Brit., pi. 32, figs. 5, 6, 1827. 



— — Flem. Brit. An., p. 384, 1828. 



— — Forb. Rep. on Egeau Invert., p. 183, 1848. 



— — Alder. Cat. Moll. North, and Durli., p. 77, 1848. 



_ _ Forb. and Hani. Hist. Brit. Moll., vol. ii, p. 293, pi. 52, fig. 6, and 



pi. S, fig. I, 1849. 



OsTEEA TUMIDA. Turt. Conch. Diet., p. 132, 1816. 

 Pecten tumidhs. Turt. Brit. Biv., p. 212, pi. 17, fig. 3, 1822. 



— — Flem. Brit. Anim., p. 384, 1828. 



— — Thorpe. Brit. Mar. Conch., p. 117, 1844. 



— — S. Wood. Catalogue, 1840. 



— — Lovvn. Ind. MoU. Scand., p. 36, 1846. 



— — G. B. Sow., Jr. Thes. Conch., vol. i, p. 57, pi. 13, figs. 27—29, 1847- 



— PYGM^us? PAi7. En. Moll^ Sic, vol. ii, p. 61, 1844, (not Go/^//'.) 



4 



