88 CARBONICOLA, ANTHRACOMYA, AND NAIADITES. 
along what lines the modern Unio has developed from a Mytiliform ancestor. 
Mycetopus from the Parand and Amazon retains the edentulous hinge, expanded 
posterior end, and subparallel margins which are so characteristic of Anthracomya ; 
Anodon also has a very strong external resemblance, but I have never seen the 
erosion of the umbones in any species of Anthracomya. The Anthracomye are, 
as a rule, comparatively rare, and form a very small percentage of the molluscan 
remains at any horizon, although in one or two beds one species may occur 
abundantly, in which case other shells are very seldom found, They had not, with 
the exception of Anthracomya Phillipsii, the gregarious habit of Carbonicola and 
Naiadites. The members of this genus are, as far as is known at present, 
absent in the Upper Coal-measures, with one exception, A. Phillipsii; and are 
chiefly found in the Middle Coal-measures of Lancashire, Yorkshire, and the 
Midlands ; in Scotland, one species (A. Scotica) occurs in the Calciferous sandstone 
beds, and another (A. modiolaris) is found in beds of the Carboniferous limestone 
group, and also in those of the true Coal-measures. The so-called Lower Coal- 
measures of North Staffordshire really belong to the middle division, the Gannister 
series being represented in this area by the lower beds only of the Biddulph 
and Cheadle basins, where alone the fossils characteristic of the Yorkshire 
and Lancashire gannister are to be found. In North Staffordshire an arbitrary 
line is taken at the Ash or Rowhurst coal, which is said to be at the top of the 
Lower Measures; but there is neither lithological nor paleontological ground 
for this division, which should be placed, I think, more correctly, if anywhere, 
above the Bulhurst seam, where shales with Aviculopecten, Goniatites, Lingula, 
and Posidonia occur in the Pottery coal-field, and at the Stinking coal shale of 
the Cheadle coal-field, where a similar fauna is also to be found. Even in 
Lancashire, beds full of Carbonicola are to be found in the Lower or Gannister 
Coal-measures. 
In a letter last year, Mr. Kirkby drew my attention to the close resemblance 
between Portlock’s Modiola Macadami and its two varieties, and M. subparailela, 
and the shells of the genus Anthracomya. I figure two specimens, kindly lent me by 
Mr, Wright, of Belfast, from the Coal-measures of Ballycastle, on Pl. XVII, figs. 1 
and 2, which seem to be characteristic of the form known as Modiola subparallela, 
which Portlock states in his observations also merge into M. Macadami. The 
original figure approaches very closely in contour to some forms of Anthracomya, 
while M. Macadami, especially the variety elongata, would seem to resemble 
Anthracomya Phillipsii. Portlock, however, goes on to say this variety approxi- 
mates to M. lingualis, Phillips, a Mountain-limestone form. The Irish specimens 
occur in beds of shale with large scales of Holoptychius Portlocki, now Archichthys 
(Strepsodus) Portlocki. 
