144 CARBONIFEROUS LAMELLIBRANCHIATA. 



Observations. — This is, perhaps, the most common of all species of ParaUelodon 

 in the Carboniferous Limestone of England, and reaches a greater size than any 

 other species. It appears to occupy an intermediate position between P. cancellatus 

 and P. Walciodorensis, being more transverse with finer radiating strias and a less 

 acute, oblique diagonal ridge than the former ; but with a stronger ridge, more 

 strongly marked radiating concentric lines, forming the characteristic reticulate 

 ornament, than obtains in the latter. In casts, even, it is possible to distinguish 

 these three forms. In the interior of P. cancellatus, PI. X, figs. 6 and 8, the 

 radiating lines are more strongly ribbed on the body of the shell ; in the interior 

 of P. reticulatus radiating lines are found on the dorsal slope only, but no trace 

 of such radiating lines to be seen on casts of P. Walciodorensis. 



The reticulate ornament of the species under notice varies much in detail in 

 different specimens ; the reticulations being much larger in the shell from Settle, 

 here figured, PI. X, fig. 4. 



It is interesting to note that de Koninck does not describe this species amongst 

 the forty-three forms which he figures from Belgian Carboniferous strata. 



The shape of the posterior end and of the posterior superior angle appears to 

 be somewhat variable ; but it is very difficult to obtain perfect examples. 



I think it probable that the Byssoarca clathrata of M'Coy is only the young 

 form of the shell under discussion, but, as the original specimen has disappeared, 

 it is impossible to speak with any degree of certainty on the subject. 



Parallelodon cancellatus, Martin, sp., 1809. Plate X, figs. 5 — 9, 9 a. 



Ascites cancellatus, Martin, 1809. Petrificata Derbieusia, pi. xliv, fig. 7. 

 Arca cancellata, Soiverby, 1824. Mill. Couch., vol. v, p. 115, pi. cceclxxiii. 



fig. 2. 

 Fleming, 1828. Hist, of Brit. Animals, p. 399. 



— Lacordairiana, de Koninck, 1842. Anim. Foss. terr. Carb. Beige, p. 119, 



pi. ii, fig. 14. 



— cancellata, Brown, 1849. Illustr. Foss. Conch., p. 190, pi. lxxvii, fig. 18. 

 Ccctjll^a cancellata, Brown, 1849. Ibid., p. 193, pi. lxxviii, figs. 14 and 16. 

 Byssoarca costellata, M l Coy, 1854. Synopsis Carb. Foss. Ireland, p. 72, pi. xi, 



fig. 36. 

 Arca cancellata, M'Coy, 1854. Ibid., p. 71. 



— — Bigsby, 1878. Thesaurus Devonico-Carboniferus, p. 299. 



— costellata, Bigsby, 1878. Ibid., p. 299. 



— cancellata, EtJieridge, 1885. Brit. Foss., part 1, Palseoz., p. 279. 



— costellata, Etheridge, 1885. Ibid., p. 279. 



