392 CARBONIFEROUS LAMELLIBRANCHIATA. 



portion of the valve in front of tliis fold is convex ; but there is a marked oblique 

 shallow compression, starting below the umbo, which becomes broader and 

 shallower as it approaches the inferior margin. Posterior to the ridge the valve 

 is rapidly compressed and flattened, and slightly expanded upwards, forming a 

 comparatively large dorsal slope, marked off above the escutcheon by a well- 

 defined erect angular ridge. The slope is divided into three almost equal parts 

 by two radiating lines more or less pronounced, which pass diagonally across it,. 

 and at each of which the valve is slightly bent on itself towards the middle line. 



Interior. — The anterior adductor muscle-scar is of fair size, pear-shaped, 

 almost marginal, and bounded on its posterior edge by a slight elevation. The 

 posterior adductor scar is well marked but shallow, rounded. It is placed just 

 below the hinge-line, remote from the posterior margin. The anterior part of the 

 hinge-line is unknown, but the posterior portion is straight, with a rolled margin. 



The pallial line is entire, remote from the margin. 



Exterior. — The surface is ornamented with very fine concentric stride and lines 

 of growth, some of which are here and there accentuated. At the oblique fold 

 these lines are suddenly bent upwards, and, as they pass, each of the two smaller 

 radiating lines on the dorsal slope becomes bent slightly forwards at a wide angle, 

 thus accentuating these lines. 



Shell thin. 



Dimensions. — A medium- sized example from Fermanagh (probably the locality 

 where the type of Portlock's shell was obtained), PI. XLII, fig. 14, measures — 

 Antero-posteriorly . .- . .43 mm. 



Dorso-ventrally . . . .18 mm. 



Lateral thickening of each valve . . .8 mm. 



Localities. — England : the Carboniferous Limestone of Lowick, and the 

 Redesdale Ironstone series, Northumberland ; in black shales below the Millstone- 

 grit of Pule Hill, near Marsden, and Hebden Bridge. Ireland : the Carboniferous 

 Limestone of Carnteel, Tyrone; Drumkeeran, co. Fermanagh; Old Abbej^ co. 

 Limerick; Ballinabintry, co. Cork; Bundoran, co. Donegal. 



Observations. — I am decidedly of opinion that two distinct shells have been 

 referred to S. tricostaivs, Portlock, sp., both possessing three radiating lines on 

 the dorsal slope. The differential diagnosis is discussed at length in my observa- 

 tions on S. striato-granulatus (p. 393). The type specimen of 8. tricostatus is 

 preserved in the Museum of the Geological Survey, Jermyn Street, and I am 

 permitted to refigure it by the kindness of the Director-General ; and other 

 specimens from the same locality are in the collection of the Geological Survey of 

 Ireland. 



This species is fairly abundant in the Redesdale Ironstone series, whence I 

 have obtained shells with portions of the exterior beautifully preserved, but I 



