MODIOLA MEGALOBA. 61 



MoDiOLA MEGALOBA, Bigsly, 1878. Thesaurus Devouico-Carbouiferus, p. 389. 

 — — Etheridge, 1888. Bristish Fossils, vol. i, Palaeoz., p. 285. 



Mtalina Fotnesiana (pars), Etheridge, 1888. Ibid., p. 286. 



Specific Characters. — Transversely and ovately triangular, of moderate size. 

 The anterior portion of the shell is represented by a small tumid lobe, well marked 

 off from the rest of the surface by an oblique constriction ; the anterior border is 

 bluntly rounded, and passes by a gradual curve into the inferior border, which is 

 directed obliquely downwards and backwards, and almost straight, making an 

 acute angle with the hinge-line. The posterior border is obliquely truncated above 

 and rounded below in young specimens, but in larger ones less truncate and more 

 rounded. 



The hinge-line is straight, equal in length to quite two-thirds of the greatest 

 length of the shell, and joins the posterior border at an obtuse angle. The 

 umbones are anterior, swollen, acute, but not raised above the hinge-line, apparently 

 contiguous, not terminal. Proceeding backwards and downwards obliquely from 

 the umbones is a gibbosity, narrow but convex, transversely at first, but expand- 

 ing as it passes downwards, and in the lower third it becomes flattened. The 

 anterior slope of this ridge is rapid, and it is bounded in front by the sulcus which 

 marks off the anterior part of the shell, the sulcus being almost linear above, but 

 becoming broader and deeper as it approaches the margin, and having an oblique 

 direction. Posteriorly and upwards the shell is gradually compressed and 

 expanded into its borders. 



The Interior. — The hinge-line is non-striate, and has the indication of a longi- 

 tudinal groove for an interaal ligament just within the hinge-edge. The anterior 

 adductor muscle-scar is small, deep, and marginal below ; and in front of the beak 

 the posterior adductor scar is large, shallow, ovate, and is placed a short distance 

 from the margin at the posterior superior angle. 



The pallial line is simple and remote from the margin. The internal surface is 

 almost smooth, but has, below and behind, obscure concentric sulci. 



Exterior .—llhe, surface of the shell is covered with fine and close lines of growth, 

 which have a direction parallel to the borders. The left valve is the more convex. 

 Shell thin. 



Dimensions. — Fig. 7, PI. II, measures — 



Antero-posteriorly . . . .30 mm. 



Dorso-ventrally at end of hinge-line . . 18 mm. 



Elevation of valve, about . . .5 mm. 



Localities. — The Coombs Limestone, Redesdale, and Sillo Burn, half-mile 

 north of Netherwood, Northumberland. Coal-measure shales, Foynes Island, 

 CO. Limerick. In shales on the shore near Kinghorn, Fife; very small form. 



