POSIDONIELLA ELONGATA. 89 



Specific Characters. — Shell very narrow, transverse, produced downwards and 

 obliquely backwards, amygdaloid with subparallel edges, acutely pointed and tumid 

 in front, flattened and rounded behind. 



The anterior edge of the valve is short, oblique, and prolonged downwards 

 and outwards, a little concave above. This border marks off an anterior surface, 

 elongate-cordate, hollow above, narrowing below to a point where the anterior 

 and inferior borders join. The inferior border is obliquely ellipsoidal, with long- 

 limbs anteriorly and posteriorly, nearly parallel. The posterior border is long 

 and straight, obliquely truncate, almost parallel with the anterior edge. 



The hinge-line is short, straight, joining the posterior border at such an 

 obtuse angle as to be often scarcely perceptible. 



The umbones are acute, narrow, swollen, terminal, and twisted forwards and 

 inwards. Proceeding backwards and downwards from the umbones is an oblique 

 swelling, the greatest elevation of which is almost central, but the slope is more 

 rapid on its posterior than on its anterior side. 



There is little or no flattened and expanded posterior-superior angle. 



Interior. — The internal surface is smooth. The anterior adductor scar is small, 

 punctate, almost apical in position, represented in the cast by a small round 

 elevation. The Yorkshire specimens show no trace of a pallial line, and the 

 posterior adductor scar is faintly indicated ; but in the Northumbrian specimen 

 the pallial line is remote from the margin and entire, and the posterior adductor 

 scar is situated low down, remote from the margin, shallow, and oval. 



Exterior. — The surface is roughened by unequally marked lines of growth, 

 which follow the general contour of the shell, and, arising from the anterior 

 surface, arch over the convexity, and terminate in the short hinge-line. Shell of 

 moderate thickness. 



Dimensions. — PI. V, fig. 16, measures — 



Greatest oblique antero-posterior diameter . . 40 mm. 



Length of hinge-line . . . .15 mm. 



Elevation of valve . . . .7 mm. 



Localities. — The Carboniferous Limestone of Settle and Hill Bolton, Yorkshire ; 

 Thorpe Cloud, Derbyshire; Coombes Limestone, Redesdale, Northumberland. 



Observations. — A series of three specimens of this very distinctive shell are in 

 the Burrow Collection of the Wooclwardian Museum, Cambridge. I am permitted 

 by the kindness of Prof. McKenny Hughes to describe and figure them. PI. V, 

 fig. 16, shows a single valve (the left), possessing the greater portion of the shell, 

 has its peculiar and characteristic shape preserved, as is at once apparent 

 on placing the specimen with the umbo pointing forwards and the hinge-line 

 horizontal. 



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