PROTOSCHIZODUS SUB^EQUALIS. 239 



situated immediately within the antero-superior angle ; the posterior, better 

 marked, is situated in the hollow of the dorsal slope, remote from the postero- 

 superior angle. The hinge has not yet been observed. 



Exterior. — The surface is almost smooth, but there are occasionally very 

 indistinct concentric lines of growth, better marked in front. Shell thin. 



Dimensions. — PI. XVIII, fig. 14, measures — 



Antero-posteriorly .... 20*5 mm. 

 Dorso-ventrally . . . .19 mm. 



From side to side . . . .13 mm. 



Localities. — England : the upper beds of the Carboniferous Limestone, 

 Castleton, Derbyshire. 



Observations. — This species is described by de Koninck in the Appendix to his 

 great work (supra cit.). In his observations on the species he says, " Cette 

 espece, qui par sa taille et sa forme trigonale, a quelques rapports avec le 

 P. Wortheni, s'en distingue par le prolongement et la forme beaucoup plus 

 anguleuse de son cote anterieur." To this I would add that P. trigonalis is more 

 gibbose, the posterior border more nearly vertical, the postero-inferior angle less 

 acute, and the inferior border less curved than in P. axiniformis. 



As far as can be ascertained at present, the two species mentioned above do 

 not occur together, either in Belgium or in Great Britain, P. axiniformis occurring 

 in Etage I, Tournai, and P. trigonalis in Etage II, Panquys. In Great Britain, 

 however, this species occurs at the top of the zone of Prodiictus giganteus, which 

 shell is considered in Belgium to be typical of the Viseen, or upper division of 

 the Carboniferous Limestone. I have obtained only one specimen of this species 

 in Great Britain, and that from the fossiliferous beds at the top of the Carboni- 

 ferous Limestone series at Castleton. 



The species is easily distinguished from P. subsequalis by its triangular form 

 having its greatest transverse diameter at the lower border, the great expansion 

 laterally of the dorsal slope, and absence of any alation at the postero-superior 

 angle. 



Pkotoschizodus subtEQCtalis, de Koninck, 1885. Plate XVIII, figs. 15 — 19. 



Protoschizodus sub^qttalis, de Koninclc, 1885. Ann. Mus. Hist. Nat. Belgique, 



vol. xi, p. 130, pi. xxii, figs. 30, 31. 



Specific Characters. — Shell somewhat transverse, small, moderately gibbose, 

 inequilateral, ovately subtriangular. The anterior end is somewhat shorter than 

 the posterior, regularly gibbose, and has an evenly rounded border, passing witli 

 a single curve into the inferior border, which is regularly but less convex. The 



