POSIDONIELLA PIRIFORMIS. 87 



Specific Characters. — Shell pyriforrn, with a slight process on one side of the 

 acute end, or obliquely subquadrate ; only moderately convex, with a peculiar 

 subspiral appearance. The anterior edge of the shell is dilated, so that there is an 

 anterior surface cordate and concave ; the posterior part is expanded and sub- 

 alate. The anterior surface is bounded by a curved line forming the anterior 

 border of each valve. This line starts from the apex of the umbo above and curves 

 outwards at first; afterwards, gradually approaching the middle line, it unites with 

 the edge of the valve, and passes gradually into the inferior border, which has a 

 semicircular curve. The anterior border is straight but oblique. The posterior 

 border is obliquely truncate, and almost straight in its upper two-thirds ; passing 

 below into the inferior border with a gradual curve. The hinge-line is straight, 

 and forms much the shortest antero-posterior diameter of the shell, joining the 

 posterior border at an angle a little greater than a right angle. 



The umbones are terminal, swollen, acute, elevated above the hinge-line, and 

 twisted inwards and forwards. They are excavated in front by the hollow 

 anterior surface, and posteriorly are not well defined from the general gibbosity 

 of the shell. The shell is swollen in front ; at first the convexity is very narrow 

 transversely, but gradually becomes broader and less pronounced as it approaches 

 the edge of the shell. The absence of the anterior end of the shell, the terminal 

 beaks, and oblique direction give the valve a twisted or subspiral appearance. 



Interior. — The hinge-line is simple and edentulous posteriorly. Anteriorly 

 the hinge is as yet unknown, but is somewhat thickened. The anterior adductor 

 scar is small, round, and pit-like ; the posterior is of moderate size, oval, situated 

 a little nearer the upper than the lower border, and submarginal. The mantle- 

 line is linear, entire, and remote from the margin. 



Surface. — The valves are marked with fine lines of growth, parallel to the 

 borders, all terminating on the hinge-line, with here and there an approach to 

 concentric undulations by the thickening and throwing into relief of some lines of 

 growth, which thus divide the smaller markings into groups. On the anterior 

 surface the lines of growth are crowded but very distinct. 



Dimensions. — PI. VI, fig. 20, the fairly large specimen, measures — 

 Greatest oblique diameter . . .43 mm. 



Length of hinge-line . . . .22 mm. 



Elevation of valve .... 8 mm. 



Localities. — The Carboniferous Limestone of Settle, Hill Bolton, Castleton, 

 and Thorpe Cloud. 



Observations. — Although a specimen of this species was figured by Phillips 

 under the name of " Perna ? " it was unaccompanied by description, and I have 

 been unable to find any record of a similar shell. Fortunately Phillips's original 

 specimen is preserved in the Gilbertson Collection of the British Museum (Natural 



