Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/678

578 rays unarticulated only for about $1⁄3$ of their length; the dorsal and anal fins are large and triangular, the dorsal being placed far back and nearly opposite the anal. The latter is slightly larger than the dorsal, but is similar to it in shape, and is not prolonged backwards in the fringe-like manner characteristic of Pygopterus.

For further details regarding the structure of this genus I must refer to my previous papers on the subject. A second species, which I have named N. gracilis, has also recently turned up in the black-band ironstone (Carboniferous Limestone series) of Gilmerton, near Edinburgh.

It thus follows that there is no Carboniferous species of Pygopterus as yet known, and that it must consequently be regarded as strictly a Permian genus, though in this case, as in others, negative evidence may be at any time overturned. I have also, in the preceding pages, endeavoured to show that a better understanding of the Carboniferous species hitherto ascribed to Amblypterus, Palæoniscus, and Gyrolepis excludes these genera also from the Carboniferous list, as far as our present knowledge goes. This is especially important with regard to the questions still pending as to the respective limits of the Carboniferous and Permian formations, both in England and in Bohemia ; for it cannot be denied that an accurate determination of genera, as well as of species, of imbedded fish-remains is essential to all safe generalization as to the aspect or distinctions of particular faunas, whether they be "Carboniferous" or "Permian," and that such generalization has been seriously impeded by the vague ideas hitherto prevalent regarding the genera discussed in the present paper. I may therefore express a hope that the observations here recorded may not be without their value, in spite of the inevitable shortcomings which the nature of the subject, and the limited opportunities of any one observer, forbid them being without.