PARALLELODON BISTRIATUS. 135 



shell; its border is bluntly and somewhat obliquely rounded, and passes with a 

 regular curvature into the inferior border, which is almost straight in its anterior 

 two-thirds, and may be slightly concave downwards about the centre ; the 

 posterior third sloping gradually upwards to meet the lower part of the posterior 

 border, and forming a narrow acutely rounded postero-inferior angle. Above 

 this angle the posterior border is long and straight, obliquely truncate from 

 before backwards and from above downwards, making a very obtuse and therefore 

 almost imperceptible angle with the hinge-line, which is straight, short, and very 

 slightly raised posteriorly, projecting very slightly in front of the umbones. 

 The umbones are small, pointed, curved inwards and forwards, and somewhat 

 raised above the hinge-line ; close, but not contiguous, and situated very far 

 forwards. Passing downwards and backwards from the umbones to the upper 

 part of the pointed postero-inferior angle is a well-defined angular ridge, 

 marking a flexure in the valve, which separates the upper and lateral surfaces. 

 Above this ridge the shell is expanded and compressed laterally, and there is a 

 broad hollow in the posterior slope on the outer side of the edge of each valve. 

 Laterally the valves are gently convex, and become gradually compressed towards 

 the lower border. 



Interior. — The position of the muscle-scars and the details of the hinge have 

 not been observed in British specimens ; but De Koninck describes the hinge, 

 from a Belgian example, as having three little very oblique and short anterior 

 teeth, and two very thin posterior teeth, parallel to the cardinal border, which, 

 however, do not occupy the whole of the space between the umbo and the 

 posterior extremity. 



Exterior. — The surface is ornamented with well-marked, regular, more or less 

 imbricating flattened concentric folds, which are wider behind than in front, and 

 covered with a very beautiful, finely reticulate, zigzag pattern, especially well 

 marked in the posterior portion of the shell. This marking seems to be confined 

 to a thin outer layer of the shell, which has frequently been accidentally removed. 



Dimensions. — PI. IX, fig. 9, a full-grown example from the Carboniferous 

 Limestone of Park Hill, near Longnor, measures — 



Antero-posteriorly . . . .19 mm. 



Dorso-ventrally . . . .10 mm. 



Laterally . . . . .8 mm. 



Localities. — England : The Carboniferous Limestone of Thorpe Cloud, Castle- 

 ton, and Park Hill, Derbyshire ; Wetton Hill and Waterhouses, Staffordshire. 

 Ireland : In the Carboniferous Shales of Benburb and Aghaloo, co. Tyrone ; 

 Carboniferous Limestone of Seafield, Wexford; Carboniferous Slate of Pouls- 

 cadden, Howth ; and Lisnapaste, Ballintra. 



Observations. — This species was doubtfully referred to the genus Pullastra by 



