SEDGWICKIA OVATA. 281 



Sedgwickia ovata, sp. nov. Plate XXVI, figs. 8 — 11 ; Plate XXVII, figs. 1 — 4. 



? Leptodomus costellatus, Kirkby, 1880. Quart. Journ. Geol. Soc, vol. xxxvi, 



p. 563. 



Specific Characters. — Shell transversely and triangularly ovate, gibbose, much 

 narrowed posteriorly by the approximation of the superior and lower borders. 

 The anterior border is semicircularly curved, passing without a break in the hinge- 

 line above and the inferior border below. The latter is gently convex, rising 

 behind to meet the posterior border at a well-marked obtusely blunted angle. The 

 posterior margin is narrow, obliquely truncate, and meets the hinge-line above at 

 an obscure obtuse angle. The hinge-line is arched in front, but extended and 

 compressed posteriorly. The urnbones are swollen, pointed, incurved, con- 

 tiguous, and directed slightly forwards, elevated above the hinge-line, and 

 situated about the junction of the anterior and middle thirds of the valve. 



The greater part of the valve is regularly swollen, and the umbo arises 

 gradually from the valve in front, but posteriorly it is limited by a well-marked 

 rounded ridge which passes downwards and backwards to the postero-inferior 

 angle of the valve ; behind this line, the valve is rapidly compressed, so that the 

 dorsal slope is hollowed and wide, and the postero-superior angle slightly 

 expanded. Valves closed all round. Ligament external, contained in a narrow 

 elongated groove parallel with the edge of the valve. 



Interior. — The anterior adductor muscle-scar is round and shallow, situated 

 near the margin at the antero-superior angle, marked off from the cavity of the 

 umbo by a ridge, and there are small linear accessory muscle-scars immediately 

 above it. The posterior adductor scar is elongate and almost obsolete, and placed 

 in the hollow of the dorsal slope. The hinge of the right valve contains a single 

 small cardinal tooth. Pallial line entire, remote from the margin. 



Exterior. — The anterior part of the valve is ornamented with many regular 

 concentric ridges and grooves, which soon become obsolete, so that the posterior 

 two thirds of the shell is smooth, and the lines of growth only faintly indicated. 

 Shell very thin. 



Dimensions. — PI. XXVI, fig. 10, measures — 



Antero-posteriorly . . . .38 mm. 



Dorso-ventrally . . . .31 mm. 



Elevation of valve . . . .13 mm. 



Localities. — England : the Redesdale Ironstone Shale, Redesdale, Northumber- 

 land. Scotland: the Scorpion-beds of Glencartholme, Eskdale, Dumfriesshire ; 

 Tweeden Burn, Cement Stone series, Roxburgh ; Encrinite-bed, east of Pitten- 

 weem Harbour, Fife. 



