AVICULOPECTEN DISSIMILIS. 71 



hinge-line straight. The umbones almost central, not elevated, triangularly acute, 

 the left the more swollen. The anterior ear with its rounded margin joining the 

 shell at an angle with the anterior border, well defined from the body of the valve. 

 The posterior ear produced along its upper margin, falcate as to its posterior 

 border, defined from the valve by a slight oblique groove. 



Interior. — The internal surface is almost smooth, but with radiating ribs near 

 the margin in the left valve, and with concentric rugae in the right. The adductor 

 muscle-scar is large, rounded, placed high up and posterior to the median line. 

 The pallial line is well marked, remote from the border. The hinge-plate is 

 narrow, elongate, and smooth. 



Exterior. — The left valve is ornamented with numerous, close, radiating ribs, 

 closely set with arched imbricated scales, the spaces between the ribs being also 

 imbricate. The right valve, on the other hand, is marked by numerous, close, 

 concentric, cremated ridges, which, in some examples, are decussated by obscure 

 radiating lines. 



The anterior ear is crossed by about five radiating ribs, diminishing in size from 

 above downwards. The upper four ribs are placed in the upper half of the ear. 

 The anterior ears are reticulate, having radiating ribs, crossed by well-marked 

 concentric lines of growth. 



Dimensions. — PI. XIII, fig. 1, a left valve, from Corrieburn, measures — 

 Antero-posteriorly . . . .56 mm. 



Dorso-ventrally . . . .61 mm. 



The internal cast of a bivalved example of similar size measures 1 5 mm. from 

 side to side. 



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

 Laws Limestone, Northumberland; Castleton, Derbyshire; Narrowdale and 

 Wetton, Staffordshire; the Red Limestone at the top of the Yoredale Scries, 

 Lr\ burn, Yorkshire; Underset and Main Limestone, Farcote Gill, Swart h Fell; 

 same horizon, Nine Standards iiigg, Westmoreland ; Poolvash, Isle of Man. 

 Scotland: the Upper Limestone series of Garngad Road; the Lower Limestone 

 series of Beith ; Hind og Glen, Dairy; Corrieburn, Pathhead, Haddingtonshire; 

 Arniston Glen, South Esk, Midlothian. Ireland: the Carboniferous Limestone of 

 Armagh and Redbarn, co. Armagh ; Cookstown, co. Tyrone; Cork and Little 

 Island, co. Cork; Clogherbrian, co. Kerry; Ballingarrane, co. Limerick. 



Observations. — Fleming gave no figures of his species, but his description is as 

 follows: — " Suborbicular, slightly lengthened, right or lower valve with numerous 

 ribs, slightly scaly. Ears nearly equal, ribbed transversely and longitudinally. 

 Left or upper valve concave, regularly marked concentrically with flat obsolete 

 ribs, which become scaly on crossing the longitudinally ribbed ears." 



For right valve read left; and for left, right, and the description is exactly that 



