300 CARBONIFEROUS LAMELLIBRANCHIATA. 



being largely due to the fact that M'Coy's specimens were all casts of the 

 interior, and Portlock's specimen is a cast of the exterior. The greater definition 

 of the posterior slope in the latter example is due to age and condition. 



M'Coy gave the name var. B. brevis to a shorter form of this species. He 

 says, " The var. B. brevis differs in nothing but the shorter figure above given, and 

 I have seen most of the intermediate grades." I do not, however, propose to 

 retain the variety. 



E. oblonga might be in a few cases mistaken for E. grandis, but the former 

 is less orbicular, less oblique, and has its borders almost parallel, and the posterior 

 end more produced and oblong. It is much less orbicular and more regularly 

 oblong than E. Lyellii. 



Edmondia Lyellii, sp. nov. Plate XXIX, fig. 4 ; Plate XXXI, figs. 1 — 5. 



Specific Characters. — Shell of moderately large size, gibbose, triangularly ovate, 

 inequilateral. The anterior end is depressed, and narrowed from above down- 

 wards, gradually compressed, the border elliptically curved, passing with regular 

 sweep into the lower margin, which is extended and only slightly convex, for the 

 greater part of its extent, but becomes more carved behind, where it passes evenly 

 into the posterior border. The latter is blunt, almost straight, and somewhat 

 narrowed by the depression of the upper border, which it meets at an obtusely 

 rounded angle. The hinge-line is arched and extended behind. The umbones 

 are large, gibbose, incurved and twisted forwards, contiguous, elevated above the 

 hinge-line, and situated in the front part of the middle third of the valve. The 

 valves are regularly convexly curved, but there is some flattening along the dorsal 

 slope and towards the posterior end of the valve. There is a well-marked groove 

 above and parallel with the hinge-line for the external ligament. 



Interior. — The anterior adductor muscle-scar is large, ovate, placed well within 

 the margin of the shell, in the hollow of the valve, at a level with the antero- 

 superior angle. The posterior adductor scar is large, shallow, and oval, situated 

 near the greater superior angle, remote from the margin. The pallial line is 

 remote from the margin and entire. The hinge-plate is thick; to the inner edge 

 of it was attached a plate, represented in casts by a narrow groove, which was 

 directed downwards and outwards. This groove is deep and wide in front, but 

 becomes narrow and shallower behind. 



Exterior. — The surface is covered with fine concentric lines of growth, which 

 are elevated into fine ridges in front and at the margin, which may appear at 

 times almost subimbricate. Here and there over the surface of the shell are 



