PROTOSCHIZODUS ORBICULARIS. 241 



Protoschjzodus orbicularis, M'Coy, 1844. Plate XVIII, figs. 21 — 23. 



Cf. Axinus orbicularis, M'Coy, 1844. Synopsis Carb. Toss. Ireland, p. G4, pi. viii, 



fig. 28. 

 „ Dolabra orbicularis, Morris, 1854. Cat. Brit. Toss., 2nd edit., p. 202. 

 „ Cucull^a orbicularis, Bigsly, 1878. Thesaurus Devonico-Carbonif., p. 305. 

 „ Schizodus orbicularis, Etheridge, 1885. Brit. Foss., vol. i, Palaeozoic, p. 290. 



Specific Characters. — Shell small, compressed, suborbicular, equilateral. The 

 anterior end is comparatively well developed, compressed, its border semicircular, 

 passing with a continuous curvature into the ventral border, which is only a little 

 less convex. The posterior border is regularly rounded, about as long as the 

 anterior; its upper limit is obscure, but there is an approach towards angulation 

 at the postero-inferior angle. The hinge-line is arcuate and short. The umbones 

 are small, triangular, poiuted, very slightly convex, not raised, and median. 

 Proceeding obliquely downwards and backwards towards the postero-inferior 

 angle is a distinct ridge, which marks off a small compressed portion of the valve 

 as the posterior slope. Elsewhere the valve is slightly but regularly curved. 



Interior. — The anterior adductor muscle-scar is small, round, and situated in 

 the hollow between the umbo and the antero-superior angle. The posterior, 

 elongate and fairly conspicuous, is placed in the hollow of the dorsal slope. The 

 pallial line is entire and remote from the margin. The hinge appears from casts 

 to be normal. There are some oblique ridges on the dorsal slope of casts, which 

 indicate hollows in the interior of the valve in this position. 



Exterior. — The surface is almost smooth even under the microscope, and the 

 shell is very thin. 



Dimensions. — Fig. 21, PI. XVIII, measures — 



Antero-posteriorly . . . .13 mm. 



Dorso-ventrally . . . .12 mm. 



Elevation of valve . . . .3 mm. 



Locality. — England: in calcareous bullions some hundred yards below the 

 third bed of Millstone-grit (Roaches), Congleton Edge, Cheshire. 



Observations. — I have referred to this species three specimens from a bed 

 some distance below the third Millstone-grit at Congleton Edge. One of the 

 specimens, fig. 22, PI. XVIII, is a cast, and gives a more perfect contour than 

 the testiferous example, fig. 21, which, owing to a loss of a small portion of the 

 lower part of the posterior end, gives an exaggerated idea of the angulation 

 of the lower portion. I have been fortunately able to identify in the casts 

 the hinge and internal characters which are typical of the genus. This species is 

 more orbicular than any other, and seems to attain to no great size. 



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