POSIDONIELLA SEMISULCATA. 93 



and downwards ; as they cross the oblique swelling on the valve they become less 

 distinct, and on the posterior wing are almost obsolete. These lines give a twisted 

 subspiral appearance to the shell. The left valve is more gibbose than the right. 



Dimensions. — PI. VI, fig. 7, measures — 



Length of hinge-line . . . ,9 mm. 



Greatest vertical diameter . . .8 mm. 



Elevation of valve . . . .4 mm. 



Locality. — In a quarry on Congleton Edge, Cheshire, south-west of Hollywood, 

 in a bed of dark shale with calcareous bullions, about 300 yards below the base of 

 the Millstone-grit. 



Observations. — This species is founded on several examples that occurred 

 in one bullion in the bed and locality stated above. I have referred them to the 

 genus Posidoniella on account of their general shape ; but, as I know of no 

 published form which in any way approaches it, I have named it after the markings 

 characterising the surface. 



This species differs from all others which I include within the genus by its 

 greater transverse diameter, and in possessing a longer hinge-line. It also has 

 not a depressed anterior surface marked off by the lines representing the anterior 

 borders of the valves. 



Casts show the anterior adductor scar to be almost terminal, and a hinge- 

 line with an elongated groove. Unfortunately the hinge-plate of the other species 

 of this genus has not yet been observed, so that it cannot be said, with our present 

 knowledge, that this condition is typical of the genus. 



In the ' Memoir of the Geological Survey : the Geology of the Country 

 round Stockport, Macclesfield, Congleton, and Leek,' mention is made (pp. 72, 73), 

 of the quarry in which the species under discussion occurs. Congleton Edge 

 forms one limb of an anticlinal fold, on which the Biddulph portion of the North 

 Staffordshire coal-field rests; and the west face of the hill shows a sequence of the 

 Millstone-grit, so-called Yoredale beds, and Carboniferous Limestone, the last 

 being thrown up as a dome by a couple of faults. The quarry south-west of 

 Hollywood occurs nearer the upper part of the hill than the base, and is in direct 

 sequence with the beds of grit forming the escarpment, so that the fossiliferous 

 horizon is not very far below the base of the Millstone-grit at this point. The 

 fauna occurring in the beds with P. semisulcata is rich in Molluscan forms, the 

 following species and genera having been obtained there by me : 



Goniatites, sp., 2. 

 OrtJioceras, sp. 

 Pleurotomaria, sp. 

 Euomphalus, sp. 

 JSIacrocheilus, sp. 



Loxonema? sp. 

 Belhroplion, sp. 

 Productus scabriculus. 

 cor a. 

 — longispinus. 



