240 CARBONIFEROUS LAMELLIBRANCHIATA. 



posterior border is somewhat elongate, obliquely truncated, and straight in the 

 upper two thirds, bluntly rounded below, where it joins the ventral edge at a 

 rounded obtuse angle. The hinge-line is short, arcuate in front, produced and 

 straight behind. The umbones are small, convex, pointed, contiguous, curved, 

 and pointing slightly forwards, and situated about the centre of the hinge-line. 

 The posterior edge of the umbo is acute, and produced obliquely downwards and 

 backwards to the postero-inferior angle, dividing the shell into a larger regularly 

 gibbose and a smaller much compressed portion, which forms the hollowed pos- 

 terior slope. The greatest convexity is about the junction of the upper and 

 middle thirds of the valve. 



Interior. — The anterior adductor muscle-scar is small and shallow; the 

 posterior rounded, comparatively large, and situated within the dorsal slope. 

 The pallial line is very faint, but entire. The hinge has not as yet been exposed. 



Exterior. — The surface is smooth, the very faintest traces only of concentric 

 markings beiug observable with a glass. Shell very thin. 



Dimensions. — Fig. 17, PI. XVIII, measures — 



Antero-posteriorly . . . .15 mm. 



Dorso-ventrally . . . .13 mm. 



Elevation of valve . . . .5 mm. 



Localities. — England : the upper beds of the Carboniferous Limestone of 

 Thorpe Cloud, Derbyshire ; Wetton, Staffordshire; and Thorpe, near Grassington, 

 Yorkshire. 



Observations. — The species P. subssqualis was founded by de Koninck on a 

 single shell from the Limestone of Nameche, stated to be in the Etage III, Viseen, 

 in the text, and Etage II in the explanation of the plate ; the latter is probably a 

 misprint, as Etage Viseen is given in the table of distribution at the end of the 

 volume. I have obtained a large number of specimens from the Limestone of 

 Thorpe Cloud, at the entrance to Dovedale, which agree so closely with the 

 figure and description of de Koninck's shell that I have no hesitation in adopting 

 his name for the British shells. 



As far as I can ascertain, P. subsequalis never seems to attain to a greater size 

 than the specimens figured in PI. XVIII, figs. 15 — 19. I have been able to ascer- 

 tain the internal characters from the cast of an interior from Thorpe by 

 Grassington, fig. 15, PI. XVIII; but unfortunately as yet the hinge-line has not 

 been exposed. 



This species comes between P. trigonalis and P. rectangularis in shape, and 

 is distinguished from the former by its less triangular and less gibbose form, and 

 by the marked alationof the postero-superior angle. The contour of the anterior 

 and inferior border is much more convex. It is, however, less circular in outline, 

 and more convex than P. orbicularis. 



