136 CARBONIFEROUS LAMELLIBRANCH1ATA. 



Portlock, in winch he was followed by M'Coy and others. The true generic 

 character was discovered by de Koninck in 1885. This talented observer had, 

 however, long before that date recognised that the shell had none of the 

 characters of PuUastra, and had referred it to Cypricardia, but curiously at that 

 time mistook the shell for a much larger example of the same genus, Parallelodon 

 squamifer (Phillips). 



.De Koninck was fortunate enough to obtain a specimen showing the character 

 of the hinge ; but he states (p. 142), " Je regrette viveraent que ce specimen ne 

 soit parvenu apres le tirage de la planche sur laquelle l'espece est representee." 



P. bistriatus is very closely allied to two other species determined by de Koninck, 

 P. elegantulus and P. normalis. There is, however, a slight difference in shape 

 between the species, and the former is stated to be devoid of the reticulated 

 ornament possessed by P. bistriatus. With regard to this character, it is entirely 

 absent in decorticated specimens, one of which is represented at PI. IX, fig. 7. 



Portlock noted the occurrence of striated and smooth forms together, and 

 whiie referring the latter to Venus elliptica?, Phillips, says (op. supra cit.), "I 

 am disposed to think they belong to the same species." Whatever the smooth 

 specimens may have been, it is quite certain that the Venus elliptica of Phillips 

 is an entirely different shell from P. bistriatus, and I therefore suspect that they 

 were semi-decorticated examples. 



M'Coy re-described the species (op. supra cit.), stating that it was " exceedingly 

 common in the Carboniferous Slate of several districts in Ireland;" but unfortu- 

 nately he gave an impossible figure, which shows a long hinge-line pointed behind, 

 with a posterior border hollowed and sulcated above, and with which the descrip- 

 tion, " posterior end obliquely subtruncate, hinge-line straight, three-fourths the 

 length of the shell," hardly corresponds. 



Of British species of Parallelodon, the one which approaches most closely to 

 P. bistriatus, and which has been confounded with it by Etheridge, is P. cingulatus, 

 M'Coy. This is markedly inequivalve, less transverse, more oblique, less pointed 

 posteriorly, with a strongly curved inferior border, and it differs in ornament by 

 having its concentric folds less flattened and often distinctly raised into ribs, and 

 by having no radiating striae. Both species occur together at Castleton and 

 Thorpe Cloud, Derbyshire. No other species of the genus Parallelodon has the 

 peculiar zigzag reticulate ornament which obtains in P. bistriatus as far as has 

 been observed up to the present. This ornament, however, is not always 

 reticulate, but is sometimes V-shaped, as on the posterior end of the shell figured 

 at PI. IX, fig. 5 a. 



I am fortunately able, by the kindness of Sir A. Geikie, to reproduce the 

 figure of Portlock's type-specimen of PuUastra bistriata preserved in the Museum 

 of the Geological Survey at Jermyn Street, PI. IX, fig. 12. It is the right valve 



