250 CARBONIFEROUS LAMELLIBRANCHIATA. 



ridges, and there is a well-marked oblique ridge posteriorly — a series of characters 

 which, taken together, are sufficient, in my opinion, to retain both species. 



Protoschizodus fkagilis, M'Coy, sp., 1844. Plate XX, figs. 6, 8, 9. 



Leptodomus feagilis, M'Coy, 1844. Synops. Carb. Fos.«. Ireland, p. 67, pi. x, 



fig. 11. 



— — Morris, 1854. Cat. Brit. Foss., edit. 2, p. 202. 



Non — — R. Etheridge, jun., 1876. Arm. Mag. Nat. Hist., ser. 4, 



vol. xviii, p. 101, pi. iv, figs. 5 — 7. 



— — R. Etheridge, 1888. Brit. Foss., vol. i, Palaeoz., p. 284. 



Specific Characters. — Shell subquadrately oval ; gibbose, inequilateral. The 

 anterior end is well developed, and has a regularly curved border which passes 

 gradually into the inferior margin, which is very gently rounded and produced 

 transversely. The posterior border is narrowed from above downwards, some- 

 what obliquely truncate and almost straight, meeting the upper and lower 

 margins at well-marked obtuse angles. The hinge-line is arched in front, 

 straight, extended and depressed posteriorly. The umbones are obtusely 

 rounded, gibbose, pointed, contiguous, elevated above the hinge-line, and situated 

 at the junction of anterior and middle thirds of the upper border. The umbonal 

 swelling rises gradually from the general convexity of the valve, and its borders 

 have an approach to sharpness only in the upper part. The dorsal slope of the 

 valve is much compressed and expanded, becoming slightly concave in section, 

 and the oblique swelling, which marks the passage of the compressed dorsal slope 

 into the general convexity of the valve, is gradually rounded. 



Interior. — I have not been able to observe the hinge or muscle-scars, but the 

 pallial sinus is entire. 



Exterior. — The surface is ornamented with regular very fine concentric striae 

 and lines of growth. Shell very thin. 



Dimensions. — Fig. 6, PL XX, the type of M' Coy's Leptodomus fragilis, 

 measures — 



Antero-posteriorly . . . .53 mm. 



Dorso-ventrally . . . .44 mm. 



Elevation of valve . . . .16 mm. 



Localities. — England : the Redesdale Ironstone, Redesdale, Northumberland. 

 Ireland : the type specimen is said to have come from the Lower Limestone, but 

 no locality is given, probably Limerick. 



Observations. — I refigure the type specimen of M c Coy's Leptodomus fragilis, 

 PI. XX, fig. 6, which is a fairly complete specimen of a left valve; but it has lost 



