410 CARBONIFEROUS LAMELLIBRANCHIATA. 



backwards, and narrowed by the descent of tbe posterior part of the upper 

 border. The postero-superior angle is slightly obtuse. The hiuge-line is much 

 arched in front, elongate, straight, and somewhat depressed posteriorly. The 

 umbones are large, elongated transversely, tumid, incurved, and pointed, elevated 

 above the rest of the valve, and placed in the anterior third of the shell. Passing 

 obliquely downwards and backwards to the postero-inferior angle is an obtusely 

 rounded swelling, which commences above as a subangular ridge, separating the 

 small but elongate and hollowed dorsal slope from the rest of the valve. A 

 blunt erect ridge passes directly backwards from the umbo to the postero-superior 

 angle, separating the dorsal slope from the wide, elongate, but shallow escutcheon. 

 The part of the valve anterior to the oblique swelling is very convex from above 

 downwards, giving rise to the appearance that the valve is rolled on itself, and 

 from before backwards the convexity is much less, and is crossed by a broad 

 shallow sulcus, Avhicli passes from the umbo downwards to the inferior margin in 

 front of the centre. 



Interior. — The adductor muscle-scars appear to be normal in position. The 

 anterior part of the hinge is not known ; the posterior part is edentulous, and 

 has a rolled margin. The inner surface of the valve is marked by obscure, broad, 

 concentric bands and sulcations, with fine, obsolete, radiating striee. 



Exterior. — The surface is almost smooth, but obsolete concentric wrinkles, 

 folds, and striis may be observed. 



Dimensions. — PL XLVII, fig. 3, a specimen from Castleton, Derbyshire, 

 measures — 



Antero-posteriorly . . . .45 mm. 



Dorso-ventraliy . . . .21 mm. 



From side to side . . . .18 mm. 



Localities. — The Carboniferous Limestone of Lowick, Northumberland ; 

 Castleton, Derbyshire; and Poolvash, Isle of Man. 



Observations. — The original type of this species is preserved in the Wood- 

 wardian Museum, Cambridge, and I am able to figure it, PI. XLVII, fig. 1, by 

 the kind permission of Prof. McKenny Hughes. It is a left valve, from Lowick, 

 Northumberland, and not a right valve, as depicted in the figure. The species is 

 not at all common, but I have collected specimens from the Carboniferous Lime- 

 stone of Derbyshire and the Isle of Man. 



The narrow, transverse, tumid shape of the shell is very characteristic, and it 

 is not easily mistaken for any other species of the genus. Notwithstanding the 

 peculiar and well-marked characters of this species, it does not seem to have been 

 recognised since M'Coy founded it, for I have not met with this name in any of 

 the lists of fossils from Carboniferous rocks. 



