30 CARBONIFEROUS LAMELLIBRANCHIATA. 



o 



growth, P. lateralis being a much younger shell. Gumming enumerates still 

 another species, which I regard as a still younger form, P. ijraciUima. Fortunately 

 this is a nomeii nudum, there being no figure or description. 



The horizontal distribution of P. Beclievi is wide, but its vertical occurrence is 

 very limited. It occurs plentifully in the Black Limestone series (Pendleside series 

 of myself and Mr. Howe, ' Quart. Journ. Geol. Soc.,' 1901, vol. Ivii) which succeeds 

 the massive White Limestone in the Midlands, South-west Yorkshire, the Isle of 

 Man, and Ireland. It was expected that this species would proA^e to be restricted 

 to, and therefore typical of, that zone ; but the species occurs in the shales below the 

 Hardraw Scar Limestone, and in connection with the Gayle Limestone in Wensley- 

 dale, and in the Carboniferous Limestone massif of Castleton, Derbyshire. But there 

 still remains the fact that the shell occurs most plentifully and very widely in the 

 zone of the Pendleside Limestone series. This series has, however, such a well- 

 marked and peculiar fauna of Cephalopoda and other species of Lamellibranchiata 

 that the somewhat earlier appearance of P. Becheri in certain localities does not 

 affect the main question of the value of the Pendleside series as a stratigraphical 

 horizon. 



Unfortunately no details of the hinge or interior of P. Bechevl have yet been 

 seen, and there is no evidence whether or no there was a byssus. It is very rare, 

 indeed, to find specimens uncrushed, but it is certain that they possessed a 

 considerable amount of convexity. 



The number and size of the concentric ridges and grooves vary considerably, 

 but shells with close frequent ridges are rarer than those whose ridges are fewer 

 and more remote. Phillips says {op. supra clt.), " In general the concentric ribs on 

 these individuals are fewer than on those which we refer to P. Becheri, though in 

 this particular much variation occurs." 



PosiDONOMYA coEEUGATA, P. Etheridfje, jim., 1873. Plate VI, figs. 1 — 5. 



Anomia corrugata {nomen mtchtm), Armstrong and Young, 1871. Trans. Geol. 



Soc. Glasgow, vol. iii, Appendix, p. 45. 

 PosiDONOMYA CORRUGATA, Armstrong, Young, and Bohertson, 1876. Cat. "West. 



Scott. Foss., p. 52. 



— — R. Etheridge, jun., 1873. Mem. Geol. Surv., expl. sheet 



32, Scotland, p. 103. 



— — — 1874. Geol. Mag., dec. 2, vol. i, 



p. 304, pi. xiii, figs. 4 — 6. 



— — Etheridge, 1878. Brit. Foss., pt. 1, Palaeozoic, p. 276. 



