POSIDONIELLA VETUSTA. 85 



and above ; equivalve. The anterior part of the shell is expanded and truncate, 

 so that there is no anterior lobe ; but there is a small lobule formed by the 

 eversion of the anterior superior angle of the valve. The surface is bounded on 

 its outer side by an obtusely angular edge, which, arising from the anterior side of 

 the umbo above, forms the anterior border of each valve. This surface is ovate- 

 cordate, flattened below, but depressed and concave above, where it excavates the 

 anterior surface of the umbo. The inferior border is almost circular, passing with 

 a regular curve into the posterior border. This is obliquely truncate, and almost 

 straight above, where it may be at times slightly emarginate. The hinge-line is 

 straight, meeting the posterior border at a slightly obtuse angle, short, pro- 

 jecting anteriorly to a slight extent, in front and below the umbones, giving 

 rise to the false appearance of a little ear in large examples. The umbones are 

 pointed, contiguous, raised above the hinge-line, and twisted inwards and 

 forwards. Passing downwards from the umbones is a convexity rapidly increasing 

 in its extent from side to side. Above, and especially towards the posterior 

 superior angle, the shell is much compressed and flattened into the hinge-line and 

 posterior border. 



Hinge linear, not thickened ; hinge-plate edentulous (on the authority of 

 de Koninck). Muscle-scars not known ; the posterior must be badly defined, 

 because, although casts are frequent, nothing is to be seen of it. Shell very thin, 

 surface covered with concentric, undulating ribs, arising as lines, crowded at the 

 anterior superior extremity ; the furrows commence immediately at the line which 

 forms the anterior border. The undulations increase in size from the umbo down- 

 wards, the ribs becoming wider and further apart. Microscopically the undula- 

 tions are covered with very fine concentric lines. Near the posterior border the 

 shell is almost smooth, the undulations gradually becoming obsolete. 



Dimensions. — Fig. 2, PL VII, measures — 



Antero-posteriorly . . . .47 mm. 



Dorso-ventrally . . . .70 mm. 



Elevation of valve . . . .15 mm. 



Localities. — England : the Carboniferous Limestone of Castleton, where it is 

 common in the quarry at the lower entrance to Cavedale ; Park Hill, Derbyshire ; 

 Bolland (Phillips), Flasby, and Todmorden, Yorkshire; Kendal, Westmoreland. 

 Ireland: Cultreagh, Clare, Kildare (Phillips). Scotland: Upper Limestone, 

 Castlecary, and Burn Anne, opposite Cessnock Castle. 



Observations. — This species was one of the few Carboniferous Lamellibranchs 

 described and figured by Sowerby. He referred it to Inoceramus, to which genus 

 its surface-markings and shape give it a marked resemblance. Phillips retained 

 the generic name Inoceramus, but thought that it should be more correctly 

 referred to Posidonia (Bronn), and in this he was followed by de Koninck. 



