POSIDONIELLA GIBBOSA. 91 



Posidoniella gibbosa, sp. nov., Hind. Plate V, figs. 12 — 14. 



Specific Characters. — Shell evenly tumid, quadrate, equivalve. The anterior 

 border of the shell is formed by the edge of the valve, which descends almost in 

 a straight line, vertically, passing with a regular curve into the inferior border, 

 which is almost semicircularly curved. The posterior border is truncate, very 

 slightly oblique, joining the hinge-line above at an obtuse angle. The hinge-line 

 is straight and prolonged forwards in front of the umbones to form a little ear, 

 which is somewhat twisted on itself. The umbones are anterior, tumid, obtuse, 

 elevated above the hinge-line, and curved very slightly forwards. The valves are 

 regularly swollen ; the line of greatest curvature is slightly oblique to the hinge- 

 line, and the slope more rapid anteriorly than posteriorly ; above, the shell is 

 slightly expanded and compressed at the posterior superior angle. Anterior to 

 the umbones is a little convex process, marked off from the general anterior slope 

 by a curved line, giving the umbones the appearance of being situated almost 

 subcentrally. 



Interior. — No details of the hinge and the muscle-scars have as yet been seen 

 in this species, all specimens yet obtained having the test preserved. 



Exterior. — The surface is almost smooth, but ornamented with a few distant, 

 accentuated, concentric strige. 



Dimensions. — Fig. 12, PI. V, a left valve, measures — 



Vertically . . . . .31 mm. 



Hinge-line . . . . .18 mm. 



Elevation of valve . . . .10 mm. 



Localities. — England : the Carboniferous Limestone of Thorpe Cloud and 

 Castleton, Derbyshire. 



Observations. — This species is founded on five specimens obtained by myself 

 from the limestone of Thorpe Cloud, Dovedale, and Castleton. It is distinguished 

 from other members of the genus by its general regular convexity, the absence of 

 an anterior surface marked off from the rest of the shell, and by the possession 

 of a small ear-like process in front of the umbo. Fig. 12, PI. V, is a specimen 

 with both valves lying open, showing that the shell was equivalve. 



The possession of an anterior ear-like process proclaims a closer affinity to the 

 Aviculidse than the other species of this genus own, but the gibbosity and 

 equivalve character of the shell at once serve to separate it from that family. Not 

 having met with this species in any of the collections I have examined, and not 

 finding any trace of a previous record, I am of the opinion that it must be very 

 rare. Four of the specimens in my collection have been obtained from the 



