310 CARBONIFEROUS LAMELLIBRANCHIATA. 



forms a continuous curve with the posterior border. The umbones are tumid, 

 short, somewhat twisted forwards, forming the highest part of the shell, and are 

 placed in the anterior quarter of the valve. 



The valves are regularly convex from above downwards and before backwards, the 

 point of greatest convexity being about the centre of the antero-posterior diameter, 

 which passes through the junction of the upper and middle thirds of the valve. 

 There is a small, narrow groove above the hinge-edge for the external ligament. 



Interior. — The muscle-scars are normal in position, and the hinge-plate is fur- 

 nished with a comparatively large ossicle, which projects outwards into the cavity 

 of the umbo. The pallial line is eutire. The internal surface is for the great 

 part smooth, but there are some irregular concentric sulci towards the lower 

 margin. 



Exterior. — The surface is covered by very fine concentric lines of growth, best 

 marked at the anterior edge of the valve, but becoming almost obsolete over the 

 posterior half of the shell. Here and there towards the lower margin are irregular, 

 shallow, concentric grooves. Shell thin. 



Dimensions. — PI. XXXII, fig. 7, measures — 



Antero-posteriorly . . . .33 mm. 



Dorso-ventrally . . . .25 mm. 



Elevation of valve . . . .10 mm. 



Localities. — England : the Carboniferous Limestone of Thorpe Cloud and 

 Castleton, Derbyshire. 



Observations. — This species occurs in fair abundance at Castleton and Thorpe 

 Cloud, and is characterised by the regularly rounded form of its posterior end 

 and its broad anterior extremity, and the fineness of the concentric strias on the 

 surface. 



It is much shorter and comparatively more oblique than E. unioniformis, to 

 which species it appears to me to have a closer affinity than to any other. 



De Koninck gives the comparative dimensions of his type specimens as : 

 " longeur 36 mm., hauteur 16 mm., epaisseur 22 mm." It will be seen on 

 referring to his figure that hauteur 16 mm. is a misprint for 22 mm. This species 

 was obtained from the Upper or Visean beds of the Carboniferous Limestone of 

 Belgium. 



Edmondia arcuata, Phillips, sp., 1836. Plate XXXV, figs. 1 — 4, 6 — 10. 



Sanguinolaria ? arcuata, Phillips, 1836. Geo]. York., pt. 2, p. 200, pi. v, 



fig. 4. 

 _ ? _ Morris, 1843. Cat. Brit. Fossils, 1st edit., p. 100. 



Sanouinolttes arcuatus, ID Coy, 1844. Syn. Carb. Toss. Ireland, p. 48. 



