242 CARBONIFEROUS LAMELLIBRANCHIATA. 



De Koninck describes a form (P. donaciformis) which has some similarity to 

 my species ; but he states that his shell is thicker transversely than P. Wortheni, 

 whereas mine is more flattened than that species, but the two species agree in 

 haviug the anterior end equal to, or slightly larger than, the posterior. De 

 Koninck's shell is stated to occur in the Waulsortian stage, and therefore at a 

 much lower horizon than P. orbicularis. 



M'Ooy described a shell under the title Axinus orbicularis in his ' Synopsis of 

 the Carboniferous Limestone Fossils of Ireland,' p. 64, but unfortunately the type 

 is lost, and nothing is now known about the specimen. The figure has very much 

 the contour of my shells, and it is represented as possessing a slightly marked 

 diagonal ridge, which also characterises mine. There is unfortunately no means 

 of establishing the identity, but I have retained the specific name because 

 description and figure agree very closely with my shells. 



Pkotoschizodus triangularis, sp. nov., Hind, 1898. Plate XVIII, figs. 11 — 13. 



? Axinus cabboxaeius, Young and Armstrong, 1871. Trans. Geol. Soc. Glasgow, 



vol. iii, Appendix, Carb. Foss. West of Scot- 

 land, p. 50. 



Specific Characters. — Shell almost equilateral, triangular, compressed. The 

 anterior end is moderately swollen almost to the anterior border, where it becomes 

 rapidly compressed into the edge of the valve. The anterior border is almost 

 straight, obliquely truncate in its upper portion, but semicircularly rounded 

 below, where it passes with regular curve iuto the inferior border, which is 

 gently but regularly convex; the posterior border is straight for the greater 

 part of its extent, obliquely truncate from above downwards and backwards; but 

 below it joins the inferior border at a bluntly rounded obtuse angle. The hinge- 

 line is very angular and short, its limits not well defined. The umbones are 

 small, elevated, pointed, and twisted forwards, almost contiguous, and sub- 

 central ; not well marked off from the valve either in front or behind. The 

 dorsal slope is very slightly compressed, because the general convexity of the 

 valve is continued almost to the edge of the shell. The valves are regularly and 

 interruptedly convex above downwards, and before backwards ; the greatest 

 convexity is subumboual. 



Interior. — The anterior adductor muscle-scar is small, shallow, and marginal; 

 the posterior elongated and submarginal. The pallial line is entire. The hinge 

 has not yet been exposed. 



Exterior. — The surface appears to be smooth. 



