Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/657

Rh how far its structure agrees with that of the original type of the genus, and have even sometimes overlooked distinctions of more than generic importance.

The species of Palæoniscus enumerated and described by Agassiz in the 'Poissons Fossiles' are referable to at least seven different types.

I. Type of Palæoniscus Freieslebeni, Ag. (Genus Palæoniscus restricted). The body is elegantly fusiform; the scales moderate, sculptured; the fins of comparatively small size; the dorsal situated opposite the interval between the ventrals and the anal; the rays of the pectoral are articulated; the fin-fulcra are small but easily recognizable. The suspensorium is very oblique, the operculum and interoperculum broad; the mandible is slender. The teeth are small, conical, sharp, and of different sizes, the smaller ones being more externally placed, but without specially prominent laniaries. The species here included are Palæoniscus Freieslebeni, magnus, macropomus, elegans, comptus, longissimus, and macroplithalmus. Palæoniscus, if limited to the species just enumerated, becomes intelligible as a genus; otherwise it seems to me, as already stated in the preliminary part of this paper, that the conception of a Palæoniscus becomes so vague that no tangible ground of distinction can be found between it and Amblypterus and many other genera of the family. It is most closely allied to Elonichthys; but from that genus it is distinguished by the small size of the fins, and by the dentition, in which the differentiation of "laniaries" has not proceeded so far. The teeth, however, are not "en brosse," as described by Agassiz, though their small size sufficiently accounts for his use of the term; probably, also, they were not very perfectly exhibited in the specimens then at his disposal; indeed in those from the German Kupferschiefer they are rarely seen at all. Agassiz's description of them as being "en brosse," and also "si excessivement petites qu'il est très-rare de pouvoir les distinguer", has been rather severely criticized by Messrs. Hancock and Atthey; it must, however, be borne in mind that the species (Egertoni) in which they correctly described the teeth as being "disposed in two distinct rows, one within the other, much in the same fashion as in Megalichthys and Rhizodopsis, but still much more like that which obtains in Pygopterus, in which the teeth are likewise arranged in two rows—one being of large laniary teeth, the other of small external ones," is not a true Palæoniscus, but is more properly referable to Elonichtliys. The passage referred to was also written by Agassiz in special reference to the species occurring in Continental Permian strata, and before he became acquainted with those Carboniferous forms with conspicuous laniaries which he somewhat incorrectly referred to the same genus.

As above restricted, the genus Palæoniscus must meanwhile be considered as limited to the Permian formation, though it has hitherto been looked upon as common also to the subjacent Carboniferous rocks. It will presently be seen that the so-called