PARALLELODON RETICULATUS. 143 



latter is truncate and straight above, and though variable, as a rule, very slightly 

 oblique downwards and backwards, but bluntly rounded below. The hinge-line is 

 straight, as long as the greatest diameter of the shell, making an angle which is 

 either a right angle, or only slightly greater than a right angle, with the posterior 

 border, and a blunted right angle with the anterior border. The umbones are 

 large, swollen, curved inward, and twisted very slightly forwards, non-contiguous, 

 raised above the hinge-line, and situated about the junction of the anterior and 

 middle thirds of the hinge-line. There is a well-marked elongated ligamental area, 

 interior to the umbones, which does not, however, extend the whole length of the 

 shell. Proceeding downwards and backwards from the upper edge of the umbones 

 to the posterior inferior angle is a rounded ridge, marking off the compressed and 

 hollowed dorsal slope from the rest of the valve, which is generally convex, but in 

 its middle third there is a flattening or compression in the region of the byssal 

 sinus. 



Interior. — The anterior adductor muscle-scar is shallow, round, and situated 

 just within the anterior superior angle of the shell ; that of the posterior adductor 

 muscle is also circular, larger than the anterior scar, and very shallow, situated on 

 the posterior slope at some little distance from the posterior extremity, and a little 

 way below the cardinal border. The pallial line is entire and distant from the 

 margin. The hinge-plate is thickened in front, and has anteriorly several (eight) 

 small oblique, shallow teeth ; posteriorly there are two to three linear teeth. The 

 inner surface of the shell is smooth except on the dorsal slope, where there are 

 obscure radiating stria?. 



Exterior. — The surface is adorned with fine, regular, concentric lines of growth 

 and stria?, which near the margin become deeper and rugose. These are decus- 

 sated by radiating stria?, which are crowded in the neighbourhood of the umbones, 

 and become more widely separated and stronger as they approach the lower border 

 of the valves. On the posterior slope the radiating stria? become so much coarser 

 as to be developed into distinct ribs, but even there they are crossed by the con- 

 centric lines of growth. In the neighbourhood of the byssal sinus the concentric 

 lines become irregular, and exhibit a small concavity directed downwards. 



Dimensions. — PI. IX, fig. 25, measures — 



Antero-posteriorly . . . .50 mm. 



Dorso-ventrally . . . .27 mm. 



Elevation of valve . . . .10 mm. 



Localities. — England : the Carboniferous Limestone of Settle and Grassington, 

 Yorkshire ; Castleton and Park Hill, Derbyshire. Ireland : the Carboniferous 

 Limestone (Lower) of Millicent, co. Cork ; Clane and Killmallock. Scotland : 

 the Carboniferous Limestone of Clatteringwell Quarry, Kingswood, Kinross ; 

 Upper Limestone, Gare ; Lower Limestone, Craigenglen. 



