420 CARBONIFEROUS LAMELLIBRANCHIATA. 



teriorly, and slightly narrowed from above downwards; very inequilateral. The 

 anterior end is very short, but deep and swollen ; its margin bluntly rounded. The 

 inferior margin is convex, especially behind, where it rises to meet the posterior 

 border, passing into it without any approach to angulation. The posterior margin 

 is narrowed, subtruncate, rounded, and joins the hinge-line at a more or less well- 

 marked obtuse angle. The superior border is nearly as long as the shell, arched in 

 front; straight, produced, and slightly depressed posteriorly. The umbones are 

 large, gibbose, incurved, pointed, contiguous, slightly raised, placed very far 

 forwards, and excavated in front by a broad ovate lunule. The escutcheon is long 

 and broad, bounded externally by erect angular ridges, which form the dorsal edges 

 of the valve. The valves are very much swollen in front, but rapidly compressed 

 behind. There is an almost obsolete, oblique, broad compression to be noticed in 

 front of the middle of the valve, best seen near the lower margin. An oblique 

 gibbosity, gradually diminishing, passes downwards and backwards from the 

 umbones towards the postero-inferior angle. 



Interior. — The anterior adductor muscle-scar is very large, pear-shaped, 

 striated from before backwards, especially below, and occupies nearly the whole 

 of the anterior part of the shell. It is bounded behind by a ridge, which leaves a 

 groove in casts. The posterior muscle-scar is large and shallow, striated 

 posteriorly from above downwards, and placed just below the upper margin, 

 occupying part of the tbickened elongate ridge, and remote from the posterior 

 border. The pallial line is markedly and deeply sinuate, the sinuation beiug very 

 narrow and angular ; its margin often marked by a row of tubercles. 



The interior of the shell is marked by concentric grooves and ridges ; but the 

 posterior upper portion is almost smooth. Parallel to, but below the upper 

 margin is a broad well-marked thickening, which becomes obsolete near the 

 posterior margin ; it is indicated by a groove in casts. 



Exterior. — From observation of American specimens the shell is covered by 

 broad concentric ribs and sulci, which become obsolete over the dorsal slope ; and 

 the ribs are often, and especially near the lower border, covered with concentric 

 striae. 



Dimensions. — PI. XLVII, fig. 7, a specimen in the collection of Mr. J. G. 

 Goodchild, from the Carboniferous Limestone near Llangollen, measures — 

 Antero-posteriorly .... 129 mm. 

 Dorso-ventrally . . . .50 mm. 



From side to side . . . .39 mm. 



Localities. — England : the Limestone of Howick, Northumberland. Wales : 

 the Upper Grey Limestone of Llangollen, and Puffin Island, Anglesea, N. Wales. 

 Scotland : the Carboniferous Series of Brunstane. Ireland : in the light-coloured 

 gritty Limestone at Donaghenry, and Donaghrisk, co. Tyrone. 



