200 CARBONIFEROUS LAMELLIBRANCHIATA. 



Cabdiomobpha Etheridgei, de Koninek, 18S5. Ann. Mus. Roy. Hist. Nat. 



Belgique, vol. xi, p. 14, pi. iii, figs. 

 16, 17. 



— elegans, de Koninek, 1885. Ibid., figs. 8, 9. 



— lata, de Koninek, 1885. Ibid., p. 13, pi. ii, figs. 11, 12. 

 ? Pachtdomtjs longus, de Koninek, 1885. Ibid., p. 26, pi. iii, fig. 15. 



— oblongus, Etheridge, 1888. Brit. Eoss., vol. i, Palseoz., p. 280. 



Specific Characters. — Shell oblique, somewhat transverse, subquadrilateral, 

 gibbose, very inequilateral, expanded posteriorly. The anterior end is very short 

 and low, compressed, especially above, where it is actually concave laterally, 

 between the base of the umbonal swelling and the edge of the valve. The 

 anterior border is semicircularly curved. The curvature hardly becomes altered 

 in degree at the anterior inferior angle, where it passes almost imperceptibly into 

 the lower edge of the valve. The inferior border is very gently curved and 

 somewhat produced, becoming more convex posteriorly, and passing without a 

 break into the posterior border, which is bluntly rounded, the degree of curvature 

 being greater at the junction with the upper and lower borders. The hinge-line 

 is arcuate, the anterior portion being at a much lower level than the posterior, 

 which is long and almost straight. The umbones are prominent, gibbose, raised 

 above the hinge-line, twisted forwards at first, and then become spirally coiled on 

 themselves, so that the pointed apices are twisted down and outwards, and partly 

 upwards. There is no lunule. The umbonal swelling is well marked off from the 

 anterior part of the shell, and rises suddenly ; posteriorly, however, it passes 

 gradually into the general convexity of the valve. The umbonal gibbosity is 

 produced somewhat obliquely backwards and downwards, but becomes lost 

 about midway across the surface of the valve. The posterior and larger portion 

 of the valve is evenly swollen, but compressed rapidly near the borders. The 

 greatest convexity of the valves is through the umbo at about the level of the 

 hinge-line. 



Interior. — The anterior adductor muscle-scar is shallow and rounded, and 

 situated immediately within the antero-superior angle. The posterior scar is 

 large, obovate, almost obsolete, and situated on the dorsal slope, remote from the 

 margins. The pallial line is entire and almost obsolete. 



The hinge-plate is edentulous. The anterior third of the posterior part is 

 formed by a bending of the valve on itself at right angles, so as to develop a flat 

 plate, which comes in contact with that of the opposite valve ; but posteriorly 

 this plate gradually becomes obsolete. Above this plate, and between it and the 

 umbo, is a narrow groove, which widens as it passes backwards, forming an 

 elongate area or escutcheon, marked off from the rest of the valve by a longitudinal 

 ridge. The ligament is not preserved in any of the specimens I have yet 



