90 CARBONICOLA, ANTHRACOMYA, AND NAIADITES. 



its junction with the posterior border. The posterior border is markedly truncate 

 from above downwards and backwards, making an obtuse angle above with the 

 hinge-line, which is rarely rounded ; the posterior inferior angle is the most pos- 

 terior portion of the shell. The hinge-line is straight, extending the whole length 

 of the upper border, directed somewhat obliquely upwards behind, so that it really 

 never is parallel to the lower border on account of its low situation at the com- 

 mencement. The umbones are obtuse, tumid, and contiguous, only slightly raised 

 above the hinge-line, somewhat excavated in front by the lunule, which is narrow 

 and elongated, and bevelled off posteriorly. Proceeding from the umbones down- 

 wards and backwards across the surface of the shell is a blunt oblique ridge or 

 swelling, which gradually becomes less conspicuous as it approaches the margin of 

 the shell near the posterior inferior angle. This oblique swelling is sometimes acute 

 and subangular, at others equally rounded. Anterior to this gibbosity the shell is 

 compressed, the compression becoming broader as it approaches the lower margin. 

 Posterior to the oblique swelling the shell is expanded and rapidly compressed, 

 especially in an upward direction, so that the posterior slope may be slightly 

 concave. 



Interior. — Pallial line entire ; anterior adductor scar marginal, shallow. 

 Posterior adductor scar obscurely marked, large, situated near the posterior 

 superior angle of the shell. I only know of one example showing the hinge, 

 and that, a left valve. The hinge consists of two portions — a short anterior 

 part, which does not appear bevelled, which stops abruptly just anterior to the apex 

 of the umbo, and is separated from the posterior part of the plate by a gap, 

 possibly for the reception of a cardinal tooth in the right valve. Posteriorly, 

 commencing just under the umbo, is a long narrow lateral tooth, formed by the 

 lower edge of the hinge-plate, above which is a shallow elongate groove ; above 

 this is the thin raised upper edge of the hinge-plate, outside which is the elongate 

 narrow groove, parallel to edge of the shell, characteristic of the genus. 



Posterior to the lateral tooth the edge of the hinge-plate is raised and bent on 

 itself, and bevelled to come in contact with a similar surface in the opposite valve. 



Exterior. — The surface is marked with fine lines and strias of growth, which 

 have a concentric arrangement parallel to the margins of the shell, more densely 

 crowded ;it the lower margin and in front; pcriostracum wrinkled. Shell thin. 



Dimensions : 



Autero-posteriorly. Dorso-ventrally. Laterally. 



Fig. 1, PI. XIV . . 44 mm. 23 mm. 18 mm. 



Fig. 9, PL XIV (Mr. Salter's type) 31 mm. 18 mm. 14 mm. 



Fig. I'.i, PL XVI (Sowerby's type) 40 mm. 15 mm. 13 mm. 



Localities. — England: Roof of the Holly Lane at 10-foot Seams, North Staf- 

 fordshire; Crawstone, Coalbrookdale ; Black Mine, Oldham ; Cowpen Colliery and 



