Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/649

Rh absence of fulcra on the fins of Amblypterus, save on the caudal, was corrected a few pages further on, though they are here said to be "si extraordinairenient petits qu'on peut à peine les entrevoir à l'œil nu". Large azygos scales in front of the median fins are also declared to exist in Amblypterus; so that the only differences remaining between that genus and Palæoniscus are the large size of the fins and the minuteness of the fulcra in the former—differences, indeed, not of a very substantial character, as will be presently shown.

As regards the structure of the fins, their rays were believed by Agassiz to be covered with scales in some species of Palæoniscus (P. Voltzii, Blainvillei), not so in others (P. Freieslebeni), and in Amblypterus —a difference which, if it did really exist, would certainly be sufficient, not to distinguish Amblypterus from Palæoniscus, but to demand the separation of the latter into two distinct genera. The scaly appearance of the fins in some so-called Palæonisci, however, is entirely deceptive, and arises solely from the form and arrangement of the minute joints of the rays themselves.

In the works of most other authors, such as Pictet, Giebel, and Quenstedt, we shall likewise fail to find any thing satisfactory as regards the discrimination of the genera in question—though Goldfuss, in 1847, pointed out that Amblypterus macropterus, Ag. (Bronn, sp.), possessed large conical teeth, its dentition being, therefore, not "en brosse," according to the previously received definition of the genus. A similar observation has also been more recently made by Messrs. Hancock and Atthey in the case of Palæoniscus Egertoni, Ag. Agassiz himself had previously described the teeth of Amblypterus punctatus as being "en cônes obtus." The only distinction we can lay hold of at all is the large size of the fins in Amblypterus and their medium size in Palæoniscus; but in this respect the greatest differences exist in the large assemblage of species which have been referred to the latter genus. And as regards this point, the vagueness of Agassiz's own ideas is well illustrated by the arbitrary manner in which he distributed certain British Carboniferous species between the two genera; for the fins of his Palæoniscus striolatus and Robisoni are proportionally just as large, and their fulcra just as minute as those of his Amblypterus neuropterus, and one of the two species which he included under the name of Amblypterus punctatus; in fact the resemblances which those fishes bear to each other are so close that their being placed in different genera is simply inadmissible.

Only by Troschel was a bold attempt made to define these genera upon strictly zoological principles, though only with partial