Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/651

Rh "Palceoniscus, Ag. (Palæothrissum, Blainv.). Plossen nur mittelgross; Strahlen stark; Kopf gewöhnlich aufgetrieben; Fulcren wie Amblypterus. Kohle bis Trias. Arten: P. Freieslebeni, Ag., P. comptus, Ag., u. a."

Now, if by "Schwanz" is meant the caudal fin with its prolongation of the body along the upper lobe, that part is certainly no shorter in the Agassizian Amblypteri than in his Palæonisci; nor, generally speaking, is the head more "aufgetrieben" in the one than in the other. In describing the Amblypterus Agassizii of Münster, Agassiz himself states that "la mâchoire supérieure forme une saillie arrondie au-dessus de la mâchoire inférieure, saillie qui résulte probablement, comme dans les Palæoniscus, du développement considerable de l'ethmoïde. Jusqu'ici j'avais cru ce caractère exclusivement propre aux Palæoniscu, n'ayant vu que peu d'exemplaires du genre Amblypterus dont la tête fut assez bien conservée pour ne me laisser aucun doute sur sa forme". Differences of dentition being altogether ignored, we are thus thrown back on the size of the fins and the number of their rays, the unsatisfactory nature of which, as diagnostic marks, I have already alluded to; in fact, if other characters are not to be taken into account, it may become a very delicate matter to decide as to whether a given small heterocercal fish has fins large enough for an Amblypterus or small enough for a Palæoniscus!

Zoologists, however, will hardly be prepared to accept the ideas of generic comprehensiveness expressed in the reunion of Rhabdolepis with Amblypterus, any more than the location by Prof. Carus of the Palæoniscoid fishes (with the exception of Cheirolepis) among the Lepidotini as a mere "subfamily;" for if Rhabdolepis be not generically distinct from Amblypterus, neither is Palæoniscus, nor Acrolepis, nor Elonichthys, and in fact, to be consistent, nearly the whole of the Palæoniscidæ would have to be merged in one huge genus. But if, on the other hand, we are to deal (according to our information) with extinct as with living forms, then the line of investigation begun by Sir Philip Egerton and by Troschel must be continued, the generic characters of Amblypterus and Palæoniscus reinvestigated and more accurately defined, their species redistributed, and, if need be, new genera instituted for those which cannot be included in the one or the other, according to the conception of a "genus" current among modern zoologists.

The results to which I have arrived, after a careful study of a very large number of specimens of Palæoniscidæ, both British and foreign, seem to me certainly to require still further modifications of the prevalent ideas respecting the characters and limits of the genera in question, as well as those regarding the extent of their stratigraphical disibution.