POSIDONTELLA SUBQUADRATA. 101 



Dimensions. — Fig. 8, PI. VII, measures — 



Antero-posteriorly . . . .11 mm. 



Dorso-ventrally . . . . 16 mm. 



Elevation of valve . . . .2 mm. 



Locality. — Shales below the Millstone-grit, Vale of Toclmorden. 



Observations. — This species is founded on thirteen specimens in the Manchester 

 Museum, Owens College. They are part of a collection, several specimens of 

 which were described and figured in the first volume of the ' Transactions of the 

 Manchester Geological Society ' by Captain Thomas Brown, and the tablets on 

 which these specimens are mounted bear the MS. names " Mytilus variabilis" and 

 " Mytilus?" in his handwriting. They are very mytiloid in shape, but evidently 

 belong to the genus Posidoniella, which is very close indeed in its general 

 affinities to Mytilus. P. variabilis is not so convex as P. Ixvis and P. Eirhmani, 

 and it is more oblique. From P. minor it is separated by its much less quadrate 

 and more rhomboidal form. 



Posidoniella subquadrata, sp. nov., Hind. Plate VI, fig. 25 ; Plate VII, fig. 10. 



Specific Characters. — Shell obliquely subquadrate, moderately gibbose, 

 especially in front and above, subulate posteriorly. The anterior border is 

 straight, and descends downwards and to a very slight extent forwards ; below, 

 it is semicircularly curved, and thus passes into the inferior border, which is 

 generally convex, but more markedly so in front and behind. The posterior 

 border is straight above and obliquely truncate from above downwards and 

 backwards, but below and where it joins the ventral border bluntly curved. The 

 hinge-line is straight and short, joining the posterior border at an obtuse angle. 

 The umbones are tumid, elevated above the hinge-line, contiguous, anterior, and 

 terminal, and very slightly twisted forwards. 



Proceeding downwards from the umbo is a well-marked gibbosity, which is 

 more marked in front, and has an abrupt descent. On the anterior side, but 

 posteriorly, the swelling becomes expanded as it descends, and is gradually lost on 

 the shell surface. Towards the posterior superior angle the shell is much 

 compressed. 



Interior unknown. 



Exterior. — The surface of the valves is covered by feebly marked folds, 

 somewhat irregularly arranged, but parallel to the contour of the shell. These 

 are very superficially decussated by almost obsolete, fine, radiating lines, more 

 marked at the posterior superior angle. 



