314 CARBONIFEROUS LAMELLIBRANCHIATA. 



Dimensions. — Fig. 12, PL XXXV, measures — 



Antero-posteriorly . . . .64 mm. 



Dorso-ventrally . . . .30 mm. 



Elevation of valve . . . .7 mm. 



Localities. — England : rare in the Redesdale Ironstone, Northumberland. 

 Scotland : in a bed of shale between two limestones at Penton Linns, river 

 Liddle, Dumfriesshire; in the Schizodus Pentlanclicus bed, Randerston, Fife } 

 Lower Limestone series. 



Observations. — This species occurs at Penton Linns in a bed of shale lying 

 between two thick limestones, associated with a very rich fauna. The following 

 Lamellibranchs occur there : — Nucnla undulata, N. gibbosa, Nuculana attennata, 

 Ctenodonta Pentonensis, Proto schizodus axiniformis, together with several species 

 of Murchisonia, Macrocheilus, Bellerophon, Orthoceras, Brachiopods, Crinoids, 

 Fenestella, &c. &c. From the fauna, I should certainly regard the bed as 

 belonging to the Carboniferous Limestone series, and probably as representing 

 the Beith, Hurlet, or Lower Limestone series of the west of Scotland ; and it is 

 probably the same as that which is exposed at Harelaw Hill and Peter's Crook 

 quarries, about a mile E. and W." respectively. 



I have been fortunate enough to obtain one specimen which has the external 

 surface of the shell removed in the neighbourhood of the hinge-line, showing the 

 outer edge of the process of the hinge-plate (a), which I have called the ossicle, 

 present in all species of the genus Edmonclia, fig. 12, PI. XXXV. This shows 

 that the plate was shelly and not cartilaginous. The outer edge of this ossicle is 

 4 mm. external to the hinge-line, and its use was probably, as Professor King 

 suggested, as a fulcrum, but it is not clear that an internal cartilage was attached 

 to it. A fair- sized shell, like Edmondia, with an edentulous hinge, would 

 necessarily need some strongly-developed muscles and ligaments to keep the 

 valves in contact ; but, compared to the size of the shells, the muscle-scars are 

 very shallow, hence the necessity for the development of some special form of 

 closing apparatus. 



I have been unable to refer the Penton specimens to any described species, but 

 they approach to E. arcuata more nearly than to any other. From this species 

 E. Pentonensis' is easily distinguished by the absence of obliquity, the short but 

 deep anterior end, and the flat oblong shape of the valves. 



Three specimens from the Redesdale Ironstone I doubtfully refer to this 

 species, but these are not perfect, and are only half the size of the Penton examples. 

 They are without the obliquity of E. arcuata, which is very common at that locality, 

 and have a deep and comparatively shorter anterior end, but it is possible that 

 they are only aberrant forms of the latter species. 



