136 CARBONIFEROUS LAMELLIBRANCHIATA. 



Interior. — Unknown. 



Exterior. — The surface is ornamented by numerous equidistant, parallel, linear 

 grooves, which become almost imbricate on the dorsal slope. These lines are 

 separated by flattened, smooth, regular spaces, parallel to the margin. 



Measurements. — Plate XXIII, fig. 9, measures : — 



Antero-posteriorly . . . 33 mm. 



Dorso-ventrally . . . 21 mm. 



From side to side (estimated from a single valve) 10 mm. 



Localities. — England : the Carboniferous Limestone of Hill Bolton, and Withgill, 

 Yorkshire. Ireland : Bruckless, co. Donegal. 



Observations. — Two specimens of this species, right and left valves, have occurred 

 to me from the Craven district of Yorkshire. Unfortunately, in the Griffith Collection 

 I cannot find the type, which was also probably a right valve, though the figure 

 would appear to represent a left valve. None of M'Coy's figures were reversed on 

 the stone, as I have pointed out before, and in each case the opposite valve to that 

 of the specimen is depicted on the Plate. 



P. elegans is more nearly allied to P. Haimeanus than to any other species of the 

 genus. It is at once distinguished from this species by the absence of fine regular 

 lines on the flat spaces between the concentric grooves and the more regular 

 character of the spaces and grooves. 



Paeallelodon noemalis, de Eoninck, 1885. Plate XXIII, fig. 8. 



Parallklodon normalis, de Koninck, 1885. Arm. Mus. Roy. d'Hist. Nat, Belg., 



torn, xi, p. 145, pi. xxi, figs. 19 — 21. 



Specific Characters. — Shell below medium size, transversely subelliptical, very 

 inequilateral, moderately convex. The anterior broader than the posterior end. 

 The anterior end short and rounded, the antero-superior angle a right angle. The 

 anterior margin rounded below, the inferior gently convex with a byssal sulcus 

 forwards. The posterior margin elliptically curved, the hinge-line straight. The 

 umbones small, not contiguous, placed very far backwards. The valve is rapidly 

 compressed on the dorsal slope, which is bounded below by a bluntly rounded, 

 oblique ridge, separating it from the convex portion of the valve. The valve is 

 constricted by a broad shallow sulcus, becoming wider as it approaches the margin, 

 indicating the byssal sulcus. 



Interior. — Normal. 



Exterior, — The surface of the valve is ornamented below with several parallel 



