PARALLELODON ANGUSTUS. 137 



concentric grooves, separating flattened, subimbricating spaces. The younger part 

 of the valve and the dorsal slope are almost smooth. 



Measurements. — PI. XXIII, fig. 8, a right valve, measures — 



Antero-posteriorly . . . .23 mm. 



Dorso-ventrally . . . .10 mm. 



From side to side (estimated from the single valve) . 12 mm. 



Locality. — The upper beds of the Carboniferous Limestone of Castleton, Derby- 

 shire. 



Observations. — A single valve, the right, of this species, lias occurred to me 

 from Castleton. It is fairly perfect and uncrushed, and shows a well-marked 

 dorsal slope, a character not described or figured by de Koninck. All other 

 species of the genus have this character, and I was at a loss to explain its absence 

 in de Koninck's diagnosis. 



The hinge-characters of P. normalis were fortunately exposed in one of 

 de Koninck's type specimens, and although lateral teeth of the hinge are described 

 they are not drawn in the normal position. P. normalis is much more transverse 

 and more cylindrical than /'. bistriatus, Portlock, sp., with the ornament of which 

 it has much in common ; but I have not discovered zig-zag or radiating lines on 

 the exterior of the valve. 



P. normalis occurs at Anseremme, Stage II in the Belgian Carboniferous Lime- 

 stone, but in England it is found with P. bistriatus (which belongs to Stage I 

 Tournaisien) and P. otifiisus, P. Lacordaireanus, P. Verneuileanus (found in Belgium 

 in Stage III Visean). The distribution of the species of this genus in Belgian 

 rocks as stated is thus different from that observed in the English Carboniferous 

 Series. 



Pakallelodon angustus, sp. nov. Plate XXIII, figs. 11 — 13. 



Specific Characters. — Shell of medium size, transversely oblong, very inequi- 

 lateral, compressed, with a long, hollow, dorsal slope. The anterior end small, 

 narrow, its margin semi-elliptical, the antero-superior angle a right angle. 

 The inferior margin long, almost straight, indented by the byssal sinus. The 

 posterior margin truncate, sinuous, joining the lower border in a bluntly rounded 

 curve. The umbonos are small, not raised, placed far forwards. Proceeding 

 obliquely downwards and backAvards from the umbo towards the postero-inferior 

 angle is a well-marked ridge which becomes less marked as it crosses the valve, 

 separating a large hollowed dorsal slope from the rest of the valve. The byssal 

 sinus is well marked. 



