POSIDOXOMYA BECHERT. 29 



diameter of the valve. The umbones are small, convex, pointed, not raised above the 

 hinge-line, and placed a little anterior to the centre of the hinge-line. The valve 

 is very gently and obliquely convex from side to side and above downwards, the 

 convexity being best marked in the subumbonal region. Tlie posterior edge is 

 flattened, and forms a narrow but deep wing. 



Interior. — Unknown. 



Exterior. — The surface is ornamented with numerous elevated concentric ribs, 

 triangular in section, separated by grooves; both ribs and grooves become broader 

 and further apart as the lower margin is approached. The ribs become obsolete 

 on the posterior wing, which appears to be striated. The shell possessed a thickish 

 periostracum, which is often seen as wrinkles, and may give rise to obscure 

 radiating striae. Shell thin. 



Dimensions. — PI. VI, fig. 14, a specimen from Budle, which has its normal 

 convexity preserved, measures — 



Antero-posteriorly . . . .82 nnn. 



Dorso-ventrally . . . .48 mm. 



From side to side (estimated) . . .4*5 mm. 



Localities. — England : in the series of Black Limestones and Shales which 

 succeed the Carboniferous Limestone (the Pendleside Series) ; Poolvash, Isle of 

 Man; Cold Coates Quarry, N.W. and Black Hall Quarry, west of Chipping; Holden, 

 near Bolton-by-Bowland ; Flashy and Sulber Lathe ; between Hill Skelterton and 

 Butterhaw ; in river Wharf at Burnsall ; Park Head Quarry, Lothersdale ; Newton 

 Gill, east of Long Preston. Above the Gayle Limestone, Raygill, Hawes, half a 

 mile east of Gawker Lathe, Yorkshire. Lancashire : Pendleside series of Dinckley 

 Hall, river Ribble, south-west of Clitheroe. About the same horizon. Mam Tor ; river 

 Dove, near Glutton Bridge, and at Tissington, and in the White Limestone, Trey 

 Cliff, Castleton, Derbyshire. Staffordshire : in the Pendleside series of Morredge 

 at Mixon Hey and Wetton. In the Culm measures of Venn and Swimbridge, Lew 

 Trenchard, and Trescot, Devonshire. Black Shales, Budle Bay, Northumberland. 



Ireland : the Upper Limestone Shales of Loughshinny and Garristown, co. 

 Dublin ; Navan and Trim, co. Meath ; Foynes Island, co. Limerick ; Kinsale Head, 

 CO. Cork. 



Observations. — This shell is generally found crushed, but a few specimens show 

 that the shell was possessed of a certain amount of convexity, PI. VI, fig. 13. 

 Early observers considered that three species of this shell could be differentiated, 

 but subsequent examination has shown that the characters relied upon for 

 specific determination Avere common to all ; for example, the tuljerculated 

 character of P. tuberculata is due to the preservation of the decussating lines, 

 which therefore appear as points on the concentric ridges. The apparent 

 differences between F. Becheri and ]\ lateralis are due to different degrees of 



