118 CARBONIFEROUS LAMELLIBRANCHIATA. 



passes downwards parallel to the anterior border, with an acute edge anteriorly, 

 at which point the shell is bent sharply downwards at an obtuse angle, so that 

 an anterior surface is formed by both valves when in contact, higher in the centre 

 than at the sides. Posteriorly and above the oblique ridge the valves are gradually 

 compressed and expanded into the borders, the oblique swelling being most 

 marked in the upper third of the shell. 



Interior. — The hinge-plate is thickened and transversely striated. The anterior 

 adductor scar is situated on the edge of the shell in front of and above the internal 

 umbo, which does not correspond in position with the external umbo on account 

 of the thickness of this portion of the shell. The position of the posterior 

 abductor and byssal muscle-scars has not as yet been observed. 



Exterior. — The surface is ornamented with a number of folds and fine striae, 

 which are arranged parallel to the borders; commencing at the anterior border, 

 which they follow, then curving over the edge of the oblique ridge, they expand 

 and form a loop, the posterior end of which passes upwards to terminate in the 

 hinge-line. 



Dimensions. — Fig. 1, PI. IV, measures — 



From umbo to inferior border . . .65 mm. 



Length of hinge-line . . . .25 mm. 



Portlock's type (PI. IV, fig. 2) measures 48 mm. in greatest 

 diameter, and has a hinge-line of 20 mm. long. 



Locality. — Redesdale Ironstone Shale, Northumberland ; Ireland, from a bed 

 of calcareous grit, Kildress River, co. Tyrone. 



Observations. — This species was founded by Portlock upon a single specimen, 

 which, being in the Geological Survey Collection, I am able to re-figure by the 

 kind permission of Sir A. Geikie, Director-General of the Geological Survey. 

 Only two specimens of this shell are known from British rocks, — one is the 

 original of Portlock's type, which at first I thought might belong to the species 

 which de Ryckholt described as Mytilus Mosensis ; and, strange to say, only one 

 specimen of that shell appears to have been discovered in Belgium, for it would 

 seem that, when this species was re-described and figured in 1885 by de Koninck 

 (op. supra cit., p. 169), the drawings and observations were made from the 

 original specimen, for he says, " Un seul specimen de cette belle espece a ete 

 decouvert dans le Calcaire Carbonifere de Vise, par le Baron P. de Ryckholt;" 

 and yet few would recognise that the two drawings had been made from the 

 same shell, those by de Ryckholt being evidently, to a very large extent, 

 idealistic. A personal examination of the original, now in the Brussels Museum, 

 convinced me of the distinct characters of the Northumbrian and Belgian shells ; 

 the latter specimen being very beautifully and concentrically striated, though 

 in shape and general aspect somewhat similar to Portlock's shell. 



