PARALLELODON CORRUGATUS. 141 



and middle thirds for the byssal opening. The posterior border is obliquely- 

 truncate from above downwards and backwards in its upper portion, but broadly 

 rounded below. The hinge-line is straight, joining the anterior border with a 

 rounded right angle, but making an obtuse angle behind with the posterior border. 



The umbones are tumid, obtuse, elongated, twisted forwards, raised above the 

 hinge-line, and widely separated. They are situated in the anterior third of the 

 valve, and are somewhat excavated in front where the umbo commences to rise 

 from the surface of the valve. Internal to the umbones, and separating them, is 

 a large ligamental area, with almost parallel borders, which are slightly concave 

 inwards, so that the area vanishes posteriorly. The surface of this area is finely 

 striated antero-posteriorly. 



Arising from the umbo, and forming its posterior and upper border, is a fairly 

 pronounced keel, which passes downwards aud backwards to the lower part of the 

 posterior border. The shell in front of this line is markedly tumid, but has a 

 well marked but shallow byssal sinus. Behind and above the ridge the valve is 

 rapidly compressed into the upper border, so as to become slightly concave. 



Exterior. — The valves are ornamented with well-marked and regular concentric 

 folds, parallel to the borders, and separated by grooves, which have about double 

 the width of the ridges. In the neighbourhood of the byssal sinus these folds and 

 grooves are all slightly arched, so as to become concave downwards. The 

 umbonal portion of the valves is almost smooth, and the markings are more 

 pronounced over the most tumid portion of the shell, and near the inferior border. 

 The folds and grooves are crossed by fine, radiating lines, more pronounced in 

 the grooves than on the ridges, and almost absent on the posterior slope. 



Interior. — Very few details have been noted. The anterior muscle-scar has 

 not been exposed. The posterior scar is large, and situated on the posterior 

 slope, well within the margin, and bounded below by the groove representing the 

 oblique ridge. The pallial sinus is situated some distance from the margin, and 

 is entire. 



The hinge is narrow in the middle two-thirds, and thickened anteriorly and 

 posteriorly. Further details are wanting. 



Dimensions. — Fig. 16, PI. X, measures — 



Antero-posteriorly . . . .40 mm. 



Dorso-ventrally . . . .20 mm. 



Depth of valve laterally . . .13 mm. 



Localities. — The Carboniferous Limestone of Settle, Yorkshire; andCastleton, 

 Derbyshire. 



Observations. — This species was founded by de Koninck, on a few specimens, 

 all more or less uncomplete, from the limestone of Vise. I have only met with 

 five examples, all of which are unfortunately fragments. Four of them are 



