SYNCYCLOXEMA SOWERBYI. L19 



hinge-plate consists of a horizontal cavity on each side of a central, small cartilage 



pit. Extending obliquely on either side of this cartilage pit are two ridges which 

 end abruptly, having in the left valve a hollow internal to them, and probably 

 corresponding to hinge-teeth. Still lower an oblique ridge separates, both in front 

 and behind, the slightly hollow flange from the concave body of the valve. 



Exterior. — The surface is almost smooth, but the microscope shows close, fine, 

 concentric lines of growth. When the upper layer of shell is removed the valve 

 appears to be covered by closely set, parallel, zigzag markings. 



Dimensions. — PI. XVIII, fig. 24, measures — 



Antero-posteriorly . . . .41 mm. 



Dorso-ventrally . . . .34 mm. 



Localities. — England : the Carboniferous Limestone of Castleton, Park Hill, 

 and Thorpe Cloud, Derbyshire ; Hill Bolton, Yorkshire ; Poolvash, Isle of Man. 

 Underset Limestone of Lunds Fell and Goodham Grill; below Hardraw Scar 

 Limestone, Mill Gill, and in Swaledale, Yorkshire. Scotland : common in the 

 Upper Limestone and the Lower Limestone series of the East and West of 

 Scotland ; Ferniehill, Gilmerton, near Edinburgh. Ireland : Yellow Sandstone of 

 Bruckless ; the Carboniferous Limestone of Little Island and Ballyrichards, 

 co. Cork ; Howth, co. Dublin ; Rathkeale, co. Limerick. 



Observations. — Syncyclonema Soiverh//i has been well studied by R. Etheridge, 

 jun., who has made out all the important features of the valve. He at once 

 recognised the importance of removing the shell from the genus Pecten, and at 

 first referred it provisionally to Amusium, afterwards to Entolium. I am not 

 certain whether both ears of the left valve are always raised, the posterior 

 appearing often straight, but it may have broken off at the transverse cartilage 

 groove, a weak spot. On the other hand, the ears of the right valve 

 are never raised, a necessary condition to permit of the opening of the valves. 

 I am inclined to think that the valves gaped at the sides, and that the 

 curious flange-like processes in front and behind had some connection with this 

 arrangement. 



The shells of >S. Sowerbyi show considerable variation, both in size and contour, 

 some being much more orbicular than the type, which is preserved in tin- Griffith 

 Collection in the Museum of Science and Art, Dublin (Pi. XVIII, tig. 2<>). This 

 specimen shows faint, irregular, oblique colour-bands nol indicated in the figure, 

 and is imperfect. 



The specimeE figured as Entolium Soiverh/iby de Koninck in L885 is evidently 

 wrongly referred to this species. The figure shows none of the peculiar characters 

 of the genus. 



There is no doubt that the Kussian shell, /'. VaMaicus. is identical with 



