378 CARBONIFEROUS LAMELLIBRANCHIATA. 



equilateral, narrowed and compressed posteriorly. The anterior end is very 

 short, swollen, and bluntly rounded. The inferior border is regularly convex, 

 becoming more so behind, where it rises to become bluntly rounded off into the 

 posterior margin. The latter is bluntly but regularly curved, and forms a 

 tolerably well-marked obtuse angle above with the hinge-line. The latter is 

 elongate but not as long as the longest antero-posterior diameter of the valve, 

 and is curved slightly upwards posteriorly. The umbones are very large, tumid, 

 much incurved, contiguous, elevated high above the hinge-line, placed in the 

 anterior quarter of the valve, but not terminal. Below the umbones in front is a 

 deep and large lunule. Passing obliquely downwards and backwards from the 

 umbo towards the postero-inferior angle is a well-marked blunt curved ridge with 

 its concavity upwards. This ridge separates the dorsal slope from the rest of the 

 valve. The dorsal slope is large, triangular, twisted on itself, almost smooth and 

 concave. It is separated from the large, elongate, deeply excavated escutcheon by a 

 prominent curved ridge. The posterior edges of the valve are depressed, in contact 

 at the bottom of the deep escutcheon. The dorsal slope is bisected by a curved 

 shallow groove, which passes from the posterior side of the umbo and terminates 

 on the posterior border. Anterior to the oblique ridge the valve is very convex ; 

 the line of greatest convexity is oblique. From above downwards the valve is 

 very much curved on itself, from before backwards the valve is regularly but 

 less rapidly convex ; the convexity is interrupted by an oblique almost obsolete 

 sulcus, which separates the short anterior portion from the rest of the valve. 



Interior. — Quite unknown. 



Exterior. — The surface of the valve is ornamented with numerous small, 

 regular, close, subangular ribs, parallel with the margin, much fewer in the 

 anterior part of the valve; but just behind the oblique sulcus others become 

 intercalated between them, and they become doubled in number. The ribs 

 become entirely obsolete as they reach the oblique ridge, and pass into fine 

 parallel strise ; these are bent upward with a blunt curve, and pass across the 

 hollow of the dorsal slope to terminate ia the upper edge of the valve. The 

 lunule is striated, the ribs of the front part of the shell being continued across it 

 to the edge of the valve in fine lines. The escutcheon seems to be smooth. 

 Shell very thin. 



Dimensions. — PL XLI, fig. 8, measures — 



Antero-posteriorly . . . .50 mm. 



Dorso-ventrally . . . .30 mm. 



From side to side . . . .31 mm. 



Localities. — England : the Carboniferous Limestone of Lowick, Northumber- 

 land. Scotland : the Lower Limestone series of Craigie, near Kilmarnock ; 

 cutting at the horizon of the Hurlet Limestone; on the Kilbirnie Railway; 



