AMUSIUM PLANICOSTATUM. 128 



Amusium tenue, de Koninclc, sp., 1885. Plate XXI, figs. 4 — G. 



Entolixtm tenue, de Koninclc, 1885. Aim. Mus. Roy. d'Hisfc. Nat. Belg., torn, xi, 



p. 242, pi. xxxii, fig. 18. 



Specific Characters. — Shell below medium size, transversely oval, equilateral, 

 feebly convex, the contour of the valve regularly curved. The hinge-line short 

 and straight. The umbones central, small, pointed. The right valve with the ears 

 not raised above the hinge-line, the anterior narrower and slightly longer than 

 the posterior ear, and more definitely marked off from the valve by an oblique 

 linear groove than the posterior ear. The left valve with the ears raised above 

 the hinge-line. The central triangular portion of the body of the valve not well 

 defined from the flange-like, expanded anterior and posterior borders. 



Interior. — The internal surface is marked by distant, radiating, shallow grooves. 



Exterior. — The surface of the valves is almost smooth, but in the left valve 

 concentric stria3 and lines of growth are seen under the microscope. The decorti- 

 eated shell shows zigzag markings. 



Dimensions. — PI. XXI, fig. 0, a left valve, measures— 



Antero-posteriorly .... 35 mm. 



Dorso-ventrally . . . .31 mm. 



Localities. — -England : a specimen in the York Museum is labelled Northumber- 

 land. Scotland : Lower Limestone series at Kirktonholm, East Kilbride. 



Observations. — I have referred four specimens from the above locality, in the 

 collection of Mr. J. Neilson, to de Koninck's species, on account of their character- 

 istic shape. 



Amusium tenue is much more transverse and regularly oval than Syncyclonema 

 Sowerbiji, to which species the specimens in question have been hitherto referred. 

 The valves are more regularly convex, especially that of the right side. The right 

 valve (PI. XXI, fig. 5) shows well-marked radiating ridges through the thin shell, 

 and therefore the species belongs to the genus Amusium. 



Amusium imanicostatum, M'Coy, sp., 1844. Plate XXI, figs. 1 — 3. 



Pecten planicostatus, APCoy, 1844. Synops. Carb. Foss. Ireland, p. 98, pi. xiv, 



fig. 6. 



Specific Characters. — Shell of medium size, transversely broadly oval, the left 

 valve only very moderately convex, the right still less so, with the anterior and 



17 



