SEDGWICKIA GIGANTEA. •• 279 



Sedgwickia gigantea, Morris, 1854. Cat. Brit. Foss., 2nd edit., p. 224. 

 Dolabra secueiformis, Morris, 1854. Ibid., p. 202. 



Sedgwickia gigantea, Bigshy, 1878. Thesaurus Devonico-Carboniferus, p. 811. 

 Cucull-Ea securiforhis, Bigshy, 1878. Ibid., p. 305. 



Sedgwickia gigantea, Kirkby, 1880. Quart. Journ. Geol. Soc, vol. xxxvi, 



p. 5S6. 

 Etheridge, 1888. Brit. Foss., pt. 1, Palaeozoic, p. 290. 

 Dolabra secdrifohmis, Etheridge, 1888. Ibid., p. 283. 



Specific Characters. — Shell large, ovately rhomboidal, convex, slightly inequi- 

 lateral. The anterior end is well developed, and its border is elliptically curved, 

 and is continuous with the upper and lower margins. The inferior border is 

 markedly convex in front, but becomes almost straight behind, where it rises to 

 meet the posterior border. The latter is narrowed by the approximation of the 

 upper and lower margins, and is almost straight, obliquely truncate from above 

 downwards and backwards, making obtuse angles with the upper and lower 

 margins. The binge-line is curved, produced and depressed posteriorly. The 

 umbones are large, subcentral, raised, tumid, pointed, incurved, and contiguous. 

 The umbones rise gradually from the surface of the valve, which appears to have 

 been evenly convex over its greater extent, but compressed posteriorly. Passing 

 from the posterior edge of the umbo downwards and backwards obliquely towards 

 the posterior inferior angle is a ridge, which, soon becomes lost on the surface of 

 the shell. 



Interior. — The anterior adductor scar is of moderate size, pear-shaped with 

 the apex prolonged upwards, and situated immediately within the antero-superior 

 angle of the shell; the posterior adductor is elongate and rough, situated in the 

 dorsal slope near the upper margin of the valve. The hinge-plate is edentulous, 

 small, and narrow, formed by a thickening of the free edge after the manner of 

 Edmondia, but less developed. The pallial line is entire. 



Exterior. — The anterior portion of the shell is ornamented with numerous 

 fairly large concentric ridges, separated by shallow sulci, which become entirely 

 obsolete about the junction of the middle and anterior thirds of the valve, from 

 which point posteriorly the valve is perfectly smooth except near the margin, 

 where faint lines of growth may at times be distinguished. Shell very thin. 



Dimensions. — The type specimen, PI. XXVI, fig. 1, figured by M'Coy, 

 measures — 



Antero-posteriorly 

 Dorso-ventrally 



A large crushed example, PI. XXVI, fig 

 Rauderstone, Fife, measures — 

 Antero-posteriorly 

 Dorso-ventrally 





61 mm. 







. 



. 40 mm. 







from 



the Calciferous 



77 mm. 



. 52 mm. 



series 



of 



