134 CARBONICOLA, ANTHRACOMYA, AND NATADITES. 
the Coal Shale of Mons should be referred to the species under discussion. His 
description is ‘‘ Coquille un peu triangular, anguleuse, trés-inéquilateral, rétrécie et 
émousée en arritre, dilatée et arondie en avant; . . . . cdté cardinal droit; région du 
ligament comprimée; région palléale renfiée,” which corresponds very well with 
that given above. Isaac Lea gives a typical figure of Naiadites modiolaris under 
the name Modiola Wyomingensis (op. supra cit.) from the Coal-measures in the 
neighbourhood of Wilkesbarre. His description is “Testa levi triangulari 
inferné compresso-alataé ; umbonibus elevatis acuté angulatis;” and he remarks, 
“This is a broad, flat species.” Scales of Palzeoniscus occurred with the shells. 
Ludwig appears to have divided this species into three, to each of which he 
gave names, apparently unaware that Sowerby and Brown had already done the 
same thing between them, and to an equal extent. I have had the good fortune 
in this case to see the original specimens now in the Museum at Dresden, and 
find that the fossils so beautifully drawn are embedded to a large extent in the 
matrix and are very imperfect. I was unable to see on what grounds Dreissenia 
laciniosa was divided from Dreissenia dilatata. The former is said to be sharply 
triangular “‘am unteren Ende dreizipfelige,” while D. dilatata is “unten stumpf 
zugerundete ;”? but in many cases the shells are so compressed and distorted as to 
render it unsafe to erect species upon them. Fig. 10, pl. xxi, op. cit., the 
drawing of D. inflata,is a pure invention, the specimen being not a fossil at all, but 
a clay-ironstone concretion. It is noteworthy of Mr. Salter’s great uncertainty of 
the real affinities of some of the Coal-measure shells that in the Geological Survey 
Memoir ‘The Iron Ores of South Wales,’ on p. 228, he gave ‘* Myalina modiolaris 
(Avicula, ‘G. Tr.,’ pl. xxxix, fig. 18), pl. u, fig. 14,” and on p. 230, “A. Pisp. 
(Avicula modiolaris, Sow.), pl. 11, fig. 14, aberrant form ;” thus referring the figure 
of one shell to two different genera. The ‘‘ diagram of the general shape and 
characters ’’ of Anthracoptera, given with his original description by Mr. Salter, 
‘Memoirs Geological Survey, Great Britain,’ ‘‘Geology of the Country around 
Wigan,” 2nd edit., Appendix, p. 38, 1862, might have been drawn from a typical 
specimen of N. modiolaris ; it possesses the emarginate posterior border which 
would become trifid if the shell were flattened, and would appear lke Ludwig’s 
Dreissenia laciniosa. This condition is very rare, being naturally absent in casts, 
and, owing to the extreme fragility of the shell at the posterior border, is rarely 
preserved except in specimens crushed flat in the shale. The presence or absence, 
then, of this emarginate posterior border is not of specific value, being present or 
absent according to the state of preservation of the fossil. In the same volume 
and on the same page from which I have just quoted Mr. Salter describes 
a new shell as Anthracoptera ? Browniana, with which he states Avicula tenua, 
referred to by Mr. Binney in the ‘ Manchester Geol. Trans.,’ vol. i, p. 161, pl. v, 
fig. 23, and described by Brown in ‘Foss. Conch.,’ p. 161, pl. Ixviii, fig. 9, is 
