92 CARBONIFEROUS LAMELL1BRANCHIATA. 



fossiliferous bed on the summit of Thorpe Cloud, at the entrance to Dovedale, a bed 

 very rich in Molluscan forms ; but scarcely any of the shells, except the Brachio- 

 pods, are to be got in anything like perfect condition. The valves are nearly all 

 separated, and lie in all positions jammed into each other, and therefore the 

 animals to whom they belonged must have been dead before the deposit was 

 formed. The valves of Brachiopods, being more firmly united, are therefore 

 more frequently found adherent. The fauna, taken as a whole, is not one which 

 would be supposed to have had the same bathymetrical distribution, and I regard 

 the bed as made up of the accumulation of washed debris from various depths of 

 the sea-floor, though the specimens exhibit no sign of having been rolled. The 

 famous fossiliferous beds of Wetton, Narrowdale, and Park Hill, which have yielded 

 to Mr. Carrington a very large and varied fauna, are of the same character, and 

 as they all occur near the top of the Carboniferous Limestone, and contain faunas 

 which have very much in common, it may be that these beds mark a definite horizon. 



Posidoniella semisulcata, sp. nov., Hind. Plate VI, figs. 7 — 11. 



e 



? Lnoceramus, Etheridge, 1866. Mem. Geol. Surv. Qt. Britain : the Geology of th 



Country round Stockport, Macclesfield, Con- 

 gleton, and Leek, p. 92. 



Specific Characters. — Shell transversely triangular, obliquely swollen in front, 

 expanded and flattened behind and above. The anterior end is obsolete, but the 

 edge of the valve forms the margin, which descends downwards and backwards at 

 an acute angle with the hinge-line, almost straight at first, but curved below, where 

 it makes with the inferior border one convex sweep. The posterior border is 

 truncate from above downwards, making an obtuse angle with the hinge-line; 

 straight above, convex below. The hinge-line is straight, nearly as long as the 

 whole of the shell. The umbones are blunt, swollen, anterior, and terminal, 

 and are continuous with an oblique swelling which passes diagonally across the 

 shell, the anterior slope of which is rounded but steep; the posterior more gradual 

 and flatter. 



Interior. — The anterior adductor scar is punctate, represented in casts by a 

 small protuberance at the extreme anterior point interior to the umbo, as 

 shown in the cast. The posterior muscle-scar is not visible; the hinge-line is 

 grooved longitudinally and parallel to the border. The hinge-plate is slightly 

 expanded and twisted on itself in front. Ligament probably lodged in the 

 longitudinal groove and internal. 



Exterior. — The shell is marked with numerous closely arranged sulcations, 

 which, arisiug from the anterior edge of the valve, separate as they pass backwards 



