380 CARBONIFEROUS LAMELLIBRANCHIATA. 



tumid ; its border rounded. The inferior border is regularly but slightly convex. 

 The posterior border is obliquely subtruncate from above downwards and back- 

 wards in the upper part, but below it is bluntly rounded into the inferior 

 border ; in older specimens the postero-inferior angle becomes more marked and 

 less curved. The hinge-line is arched in front, but straight or slightly concave 

 posteriorly. The umbones are large, tumid, incurved, pointed, contiguous, 

 elevated above the rest of the valve, and situated in the anterior fourth of the 

 shell. The lunule is large, long, and deep. The escutcheon is much excavated, 

 elongate, and of large size. The valves are convexly curved; but there is a slight 

 sinus or compression (narrow above, but becoming broader as it nears the inferior 

 margin), which commences below the umbones and may be represented by a 

 slight sinuosity at about the centre of the inferior border. There is no oblique 

 ridge, but the valve curves into the dorsal slope along an oblique line, which 

 passes from the umbo to the postero-inferior angle. The dorsal slope is much 

 compressed and hollowed, and is bounded above by an erect angular ridge, 

 slightly curved, becoming concave on its inner aspect, which separates it from the 

 escutcheon. An obscure groove passes across the dorsal slope from the umbo to 

 the posterior margin. 



Interior. — The anterior adductor muscle-scar is small and marginal ; the 

 posterior large and shallow, and placed immediately below the hinge-line, remote 

 from the posterior end. The pallial line is entire. The hinge is edentulous and 

 narrow. The interior of the valve is marked by fairly deep concentric grooves 

 and ridges, which are wanting in the hollow of the dorsal slope. 



Exterior. — The surface is ornamented with concentric rounded folds and 

 sulci, which have the following arrangement : — Commencing at the anterior edge 

 of the valve, the ridges are few and separated by comparatively wide sulci, as a 

 rule regular, but one or two fresh ones may arise independently, or a process 

 may join two ridges. As the ridges pass over the small compression, each one 

 bifurcates, or has a new ridge intercalated between it and its successor, each 

 being about half the size of the undivided ridge. These narrow ridges then pass 

 regularly across the convexity of the shell ; but as they approach the dorsal slope 

 they reunite and pass upwards across the dorsal slope, becoming gradually 

 obsolete. In the lower part of the valve the ridges break up into bundles of fine 

 striae, which curve upwards, so that the posterior part of the dorsal slope is finely 

 striate and at times subimbricate. Shell thin. 



Dimennions. — PI. XLIV, Fig. 3, a specimen from the Redesdale Ironstone, 

 measures — 



Antero-posteriorly . . . .41 mm. 



Dorso-ventrally . . . . 29 mm. 



From side to side . . . .23 mjn. 



