68 CARBONIFEROUS LAMBLLIBRANCHIATA. 



it is obscurely ribbed, the ribs being broad and flat, separated by narrow grooves. 

 The hinge-plate is narrow and long ; starting from a point at each extremity, it 

 very gradually widens as far as the middle line, apparently quite smooth. The 

 adductor muscle-scar is large, ovate, and placed high up in the valve, but posterior 

 to the median line. 



Exterior. — The surface is adorned with many simple, unequal, smooth, broad, 

 flattened, radiating ribs, separated by almost linear grooves. The two valves are 

 very similar. The anterior ears are obscurely radiate; the posterior ears have 

 many radiating ribs, but exhibit two or three stronger ribs near the upper border, 

 the whole being concentrically striated. 



Dimensions. — PI. XII, fig. 1, the type specimen, measures — 



Antero-posteriorly . . . .55 mm.+ 



Dorso-ventrally . . . .64 mm. 



Localities. — England: the upper beds of the Carboniferous Limestone of 

 Castleton and Park Hill, Derbyshire ; Hill Bolton and Settle, Yorkshire ; Pool- 

 vash, Isle of Man. Ireland : Carboniferous Limestone of Ballintrillick, Bundoran. 



Observations. — I am convinced that Pecten tabulatus, M'Coy, the type of which 

 is a portion of a right valve showing the ears, is the right valve of A. planoradiatus 

 of the same author. I re-figure the specimen (PI. XII, fig. 2). M'Coy 's descrip- 

 tion reads : " Surface of the shell with about fifteen flat ribs, separated by very 

 narrow, deep sulci." I regard the flattened broad ribs and the large posterior ears 

 as very distinctive specific characters. The type of A. planoradiatus, M'Coy, is in 

 the Woodwardian Museum, Cambridge, and I am permitted to re-figure it (PI. XII, 

 fig. 1). It is a left valve, which has lost part of its posterior ear and much of the 

 valve, and therefore does not give a correct idea of the length of its hinge-line. 

 PI. XII, fig. 3, represents a left valve more perfect in this respect, and shows 

 that the convexity of the lower border is continued until it meets the hollow 

 separating the body of the valve from the posterior ear, and is not as is shown in 

 M'Coy's original drawing. I have been able to examine the hinge-plate, and in 

 this character A. tabulatus agrees with A. semicostatus, Portlock, sp. The right 

 valve differs from the left in being somewhat flatter and having the anterior ear 

 deeply slit for the byssus. 



Mr. R. Etheridge, jun., describes the broad flattened ribs of this species as 

 being " covered by concentric, somewhat irregular, wavy, more or less imbricating 

 scales," and also as having colour-markings of a " widely zigzag pattern." I have 

 not been able to recognise these characters in any of the numerous specimens of 

 A. tabulatus which have passed through my hands, and think that a shell of some 

 other species must have been mistaken for the one in question. 



