120 CARBONIFEROUS LAMELLIBRANCHIATA. 



an anterior border, which meets the hinge-line above at an acute angle ; passing 

 downwards and backwards, with a slight concavity, just above its centre it passes 

 into the inferior border, which is bluntly rounded, the posterior limb of the curve 

 being the more obtuse. The posterior border is almost straight or slightly convex, 

 subparallel to the anterior border ; obliquely truncate from above downwards in the 

 upper portion, and rounded below, making an angle only a little greater than a right 

 angle with the hinge-line above. The latter is straight, and is equal to the greatest 

 transverse diameter of the shell, which narrows slightly on both -sides from above 

 downwards. The umbones are acute and terminal in testiferous examples. The 

 shell is regularly convex ; the greatest gibbosity being above and in front, a little 

 below the hinge-line. In front the shell is bent at an obtuse angle from the 

 oblique line of greatest gibbosity in the upper two-thirds, forming the anterior 

 surface of the valve. The posterior part of the shell is compressed and expanded 

 into its borders. 



Interior. — The hinge-line is typically thickened and striated. The anterior 

 adductor scar is shown, by the presence of a circular raised patch, in casts to be 

 on the thickened edge of the shell below and in front of the internal beak (i. e. of 

 the cast). The posterior adductor scar is very large and oval, and situated some 

 way within the margin, equidistant from the upper and posterior borders, its 

 inferior edge being at the middle point of the dorso-ventral diameter. 



The internal umbo does not correspond with the external, being not terminal 

 in the cast ; and is marked off from the rest of the cast in front by a well-developed 

 groove. The pallial line is at a considerable distance within the shell. The cast 

 shows that the anterior edge of the shell was much thickened, and finely striate in 

 its upper half. 



Exterior. — The surface of the shell is ornamented by a large number of close 

 subimbricating lamellae, at times almost rugose, which have a direction parallel to 

 the margins. Shell of moderate thickness. 



Dimensions. — Fig. 11, PI. IV, a cast, measures in greatest 



oblique diameter . . .33 mm. 



Hinge-line 

 Elevation 

 Fig. 12, a testiferous specimen, measures 

 Hinge-line 

 Elevation . 



Localities. — England : Carboniferous Limestone, Bolland, Hill Bolton, York- 

 shire ; shales below the Millstone-grit of Congleton Edge, Cheshire. Scotland : 

 Carboniferous shales of the Calciferous-sandstone series, Eskdale. 



Observations. — I have little doubt in referring the shells figured to de Koninck's 

 species M. peralata t of which he gives three figures, one being a cast; but in the 



21 mm. 



6 mm. 

 33 mm. 

 21 mm. 



6 mm. 



