312 CARBONIFEROUS LAMELLIBRANCHIATA. 



Localities. — England: the Redesdale Ironstone of Redesdale and Bellingbam, 

 Northumberland. 



Dimensions. — PI. XXXV, fig. 3, a medium-sized example, measures — 

 Antero-posteriorly . . . .42 mm. 



Dorso-ventrally . . . .20 mm. 



From side to side . . . .16 mm. 



Observations. — The type of this species, described under the name Sangaiiw- 

 laria ? ' arcuata by Phillips, is stated to have come from Harelaw, Northumberland. 

 There is a locality named Harelaw Hill Quarry in Scotland, just over the border, 

 about a mile west of Penton, which yields Carboniferous Lamellibranchs, and which 

 may possibly be the locality whence the original of this shell was obtained. Pro- 

 fessor Lebour thinks that Harelaw may be a misprint for Hareshaw (op. supra cit., 

 p. 126), which is quoted by d'Orbigny as a locality for Carboniferous fossils in 

 Northumberland. E. arcuata is a fairly common fossil at the old ironstone mines 

 of Redesdale, occurring chiefly in the form of casts, which show the details of the 

 interior very well, but casts of the exterior are also to be found in a band of 

 shelly ironstone which occurs in the series. 



This species differs from most of the others of the genus in the shape and 

 position of the anterior adductor muscle-scar. This is very far inside the margin, 

 large, surmounted by small accessory scars, and situated quite in the umbonal 

 hollow. E. scalaris, a very different shell, is the only other species of the genus 

 where this arrangement obtains. 



The elliptically pointed end is very characteristic, being proportionally much 

 longer than obtains in any other species of the genus, and this character serves 

 at once to distinguish the shell from E. Pentonensis, which has a short but deep 

 anterior end. 



PI. XXXV, fig. 7, is a fine cast of both valves, and shows the relation of the 

 hinge-line to the slits on each side, a and b, which received the sharp extended 

 ridge (ossicle) or outer edge of the hinge-plate, and c c, the hollows for the leaf- 

 like thickening of the posterior part of the roof of the umbonal cavity. There 

 was not any space between this process and the shell, and its function probably 

 was merely to strengthen the base of origin of the hinge-plate and its peculiar 

 process which projected outwards into the cavity of the umbo. How far these 

 processes extended outwards from the hinge-plates of this species is well seen in 

 this specimen, because, being a cast, the actual edge of the hinge occupied the 

 grooves immediately on each side of the median line, which are seen to be internal 

 to and below the ossicle. The depth of the process of shell in the roof of the 

 umbonal cavity varies ; in some specimens it is only just visible, though the line 

 bounding its outer edge may be sharply defined, and in a few examples the line 

 passes across the umbo to its anterior edge. I do not think that this space could 



