384 CARBONIB'EROUS LAMELLIBRANCHIATA. 



obliquely truncate from above downwards and backwards, the postero-superior 

 angle well marked and obtuse. The binge-line is gently arched in front, produced 

 and straight behind. The umbones are tumid, incurved and pointed, elevated 

 above the hinge-line, and placed in the anterior fourth of the valve. The lunule 

 is long and narrow, and slopes downwards and forwards. The escutcheon is long 

 and broad, bounded externally by a curved erect ridge, which passes from the 

 posterior edge of the umbo to the postero-superior angle. A very broad bluntly 

 rounded swelling is continued downwards and backwards from the umbo to the 

 postero-inferior angle, separating a depressed and hollowed dorsal slope from the 

 rest of the valve. 



The shell is very much curved on itself from above downwards, and much more 

 gibbose posteriorly than in front. There is a well-marked narrow constriction 

 starting at the umbo, and passing directly downwards to the inferior margin. 



Interior. — Unknown . 



Exterior. — The surface is adorned with well-marked concentric grooves and 

 ridges, narrow and close in the umbonal region, but becoming broader and further 

 apart as they approach the margin. The folds and grooves terminate abruptly 

 along a line at some small distance in front of the oblique swelling, but are 

 represented on the dorsal slope by flattened broad ridges, which become almost 

 obsolete as they reach the upper border. Each concentric ridge is interrupted at 

 the junction of the anterior and middle thirds of the valve, and is narrowed to a 

 vanishing point, the anterior and posterior portions overlapping slightly and 

 inclined to each other at a wide angle, the posterior limb being uppermost. 

 Occasionally complete interruption does not occur, in which case there is a 

 V-shaped bend in the ridge at the same position on the valve. 



Dimensions. — PI. XLII, fig. 10, from the Limestone of Thorpe Cloud, 

 Derbyshire, measures — 



Antero-posteriorly . . . .37 mm. 



Dorso-ventrally . . . .21 mm. 



Elevation of valve .... 8 mm. 



Locality. — England : the Carboniferous Limestone of Thorpe Cloud, 

 Dovedale, and Castleton, Derbyshire. 



Observations. — I have erected this species on five specimens from the shell- 

 beds of Thorpe Cloud, all of which exhibit the peculiar arrangement of the 

 concentric grooves and ridges described above. It will be noted that there is an 

 approach to this condition of things in other species of the genus, e. g. 8. 

 variabilis and S. arr/idiis. 8. interriiptus is much more transverse than either of 

 these species, and has, in addition to the interruption, the concentric grooves and 

 ridges much more strongly marked and fewer in number than in any other 

 species. 



