194 CARBONIFEROUS LAMELLIBRANCHIATA. 



so materially in one character which has been considered by systematic con- 

 chologists to be of sufficient importance that a twofold division of the Lamelli- 

 branchiata has been based on the presence or absence of this one character. 

 American palaeontologists have latterly referred several species to Yoldia from 

 external characters only. This genus, however, possesses a very deep pallial sinus ; 

 and I have preferred to retain these forms with Nuculana, because a comparison of 

 British and American forms has convinced me that they are often generically 

 and specifically identical. In Great Britain a fair number of interiors have been 

 obtained and studied, especially in the Scotch collections, which show no traces of 

 a pallial sinus. I think, therefore, the reference to Yoldia should no longer be 

 accepted. 



Meek observes in his remarks on Yoldia subscitula, 'U.S. Geol. Surv., Final 

 Report on Nebraska,' p. 206, " Of course it is only placed provisionally in the 

 genus Yoldia, its internal characters being unknown." 



I have exposed the cast of Yoldia Knoxensis from the calcareous iron-ore 

 between coals " 2 and 3," Knox Co., Illinois, and find that the pallial line is entire, 

 and that the two small protrusions on the umbo in casts, with a hollow between, 

 are present as in British forms (PL XV, figs. 8 — 10, 10a). 



Two of the six shells described by Prof. Phillips as Nucula (op. cit.) belong to 

 the genus Nuculana, namely, iV". claviformis ? and N. brevirostris. The latter species 

 only is retained, the former being without doubt synonymous with N. attenuata of 

 Fleming. 



Of the species described by M'Coy (op. cit.) as Nucula, five certainly must be 

 placed under Nuculana. One, N. birostrata, has disappeared. I believe that the 

 figures and descriptions of N. clavata and N. longirostris haye been interchanged, 

 and that these names are synonymous with shells previously described by Fleming 

 and Portlock. N. leiorhynchus, I think, is probably the young of N. attenuata, but 

 the original specimens are so poor and badly preserved that it is difficult to speak 

 with certainty in the matter. N. stilla is therefore the only species of M' Coy's 

 which is here placed under Nuculana. 



M'Coy curiously confounded the anterior end of the shell with the posterior in 

 most of his descriptions of the species of this genus, and notice must be taken of 

 this fact when reading his descriptions ; in the description of JV. clavata this, 

 however, is not the case. 



Mr. R. Btheridge, jun., has described four species of Nuculana as new, — 

 N. Sharmani, N. Traquairi, N. intermedia, and N. Youngi. I am able to retain the 

 first only, though possibly this may be synonymous with the lost JV". birostrata, 

 M'Coy. The second species is no doubt identical with N. stilla, M'Coy. The third is 

 the young of N. brevirostris, Phillips ; and N. intermedia is undistinguishable from 

 the N. Isevistriata of Meek and Worthen, whose name, therefore, takes precedence. 



