PROTOSCHIZODUS AXINIFORMIS. 229 



Schizodus carbonarius, Bigsby, 1875. Thesaurus Devonico-Carbonif., p. 311. 



— deltoideus, Bigsby, 1875. Ibid., p. 311. 



— AXiMFORiris (pars), Etheridye, 188S. Brit. Toss., vol. i, Palaeoz., 



p. 290. 



— carbonarius (pars), Etheridge, 1888. Ibid., p. 290. 



— deltoideus, Etheridge, 1888. Ibid., p. 290. 



— depuessus, Etheridge, 1888. Ibid., p. 290. 



Protoschizodus Wortheni, de Koninclc, 1885. Ann. Mus. Hist. Nat. Belgique, 



vol. xi, p. 129, pi. xxii, figs. 4, 

 5, and 9; pi. xiii, figs. 12—16. 



Specific Characters. — Shell triangularly ovate, slightly inequilateral, com- 

 pressed near the margins, moderately gibbose above; transverse and dorso- 

 ventral diameters almost equal. The anterior border is sharp, the margins of the 

 valves being thinned and projecting slightly forwards, and almost semicircularly 

 curved, passing into the inferior border without a break. The latter is regularly 

 curved, but the degree of curvature is less than that of the anterior border. The 

 posterior border is compressed and projecting, almost straight, obliquely truncate 

 from above downwards and backwards, and joins the lower margin at a blunted 

 obtuse angle. The hinge-line is short and arched, its extent not very clearly 

 indicated either in front or behind. The umbones are tumid, pointed, incurved, 

 contiguous, elevated above the hinge-line, and situated in front of the centre of 

 the shell. 



Anteriorly the umbonal swelling is not well marked on the surface of the 

 valve, but posteriorly a well-defined and fairly acute ridge passes from the upper 

 point of the umbo obliquely downwards and backwards, terminating at the 

 postero-inferior angle. In front of the ridge the valves are equally and regularly 

 curved, but above it the dorsal slope is very rapidly compressed, so as to become 

 concave both from above downwards and before backwards. 



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

 close to the margin at the extreme base of the umbonal swelling. The pos- 

 terior scar is oval, and situated in the hollow of the dorsal slope close to the 

 margin. In casts some irregular low ridges pass upwards from the scar towards 

 the umbo. The pallia! line is entire, strongly marked, and remote from the 

 margin. 



The surface of casts is marked with obscure and irregular depressions on the 

 dorsal slope. 



The hinge has not yet been exposed in British specimens, but de Koninck has 

 figured specimens (under the name P. Worth eni) which are perfectly normal, the 



hinge having the formula * ' ' 



K. 0, 1, 0. 



Exterior. — The surface is marked with fine and regular concentric lines of 



