What is an Amhlypterus? What is a Palæoniscus? How do we distinguish them? What special reasons have we for referring any of our smaller Carboniferous fishes to the one or the other genus? These are questions to which, I fear, few collectors of Carboniferous fossils could offer very definite answers, and for the very good reason that the definitions of these two genera, which are found in the works usually consulted by palæontologists, are, it must be owned, of a somewhat unsatisfactory nature.
Gyrolepis and Pygopterus also are terms frequently met with in lists of fossils from British Carboniferous localities. But by what characters do we distinguish Gyrolepis as a genus? Or what are the special marks which justify us in assuming any of our larger Carboniferous Palæoniscidæ to be generically identical with the Pygopteri of the Magnesian Limestone and Kupferschiefer? Here, again, we shall find our subject enveloped in an obscurity which can only be dispelled by fresh and careful original observation in a field which, since the days of the illustrious Agassiz, has been comparatively little trodden.
The present communication embodies the results of my own recent investigations into these subjects, though there is room and need for much additional inquiry, as is self-evident from the nature of the remains with which we have to deal.
Amblypterus and Palæoniscus.
The definition of Amblypterus given by Agassiz in his "Tableau synoptique" is as follows:—
"Toutes les nageoires très-larges et composées de nombreux rayons, P. très-grandes; A. large; D. opposée a l'intervalle entre les V. et l'A.; point de petits rayons sur le bord des nageoires, excepté au lobe supérieur de la queue. Ecailles médiocres"[1].
Of Palæoniscus, on the other hand:—
"Toutes les nageoires médiocres, de petits rayons sur leurs bords; D. opposée à l'espace entre les V. et l'A. Ecailles médiocres; quelques espèces en ont d'assez grandes, et le corps plus large et plus court que les autres. Il y a toujours de grosses écailles impaires en avant de la D. et de l'A"[2].
Both genera are elsewhere stated to have the teeth "en brosse extrêmement fine" or "en brosse"[3]. The statement as to the
