Dictyopyge, &c), as they do not show a complete heterocercy, but only an indication or approximation to it"[1].
A glance at the beautiful plate by Mr. Dinkel, with which Mr. Kirkby's paper in the 'Quarterly Journal' is illustrated, is in itself quite sufficient to raise in one's mind the gravest doubts as to the accuracy of the position assigned by that author to the little fishes in question. However, having been, by the kindness and liberality of the Earl of Enniskillen, furnished with the loan of a beautiful series of specimens, and having also examined those in the British Museum and in the museum of the Royal Dublin Society, I am now in a position to go into the question more in detail, and with the result of finding the decision so briefly expressed by Dr. Lütken most fully substantiated.
The three species described by Mr. Kirkby are very like each other, save in the general contour of the body; so that the following observations, though principally made on specimens of Palæoniscus varians, will apply also to the other two as far as essential points of structure are concerned.
The caudal fin of P. varians so closely resembles that of Ischypterus that it would, indeed, be impossible to draw any generic distinction between them from that part alone. It is, compared with that of Palæoniscus, short and feeble, few-rayed, nearly symmetrical in external outline, and hardly cleft; the rays of the upper lobe gradually diminish in length towards its extremity. The caudal body-prolongation is, as in Ischypterus, much reduced, becoming very rapidly narrow and delicate, though its scales may be traced nearly to the extremity of the short upper lobe of the fin. The caudal fin, however, is not the only one which shows a marked deviation from the Palæoniscus type. It at once strikes the eye that the fulcra in front of the dorsal and anal fins are fewer in number, set at lower angles, and very much larger and stronger than in any of the Paæeoniscidæ—that they are, in fact, proportionally nearly as powerfully developed as in Ischypterus, though they diminish in size more gradually, from the origin to the apex of the fin, than in the latter genus. Behind the margin of strong fulcra only about ten rays are counted in the dorsal, and eight in the anal; these are rather distant save just in front, and for a considerable distance show no transverse articulations. The paired fins are very small and few-rayed; and in like manner the fulcra along their margins are strong beyond any thing met with in the Palæoniscidæ.
The osteology of the head shows a still more marked deviation from the type of structure in the Palæoniscidæ. The opercular bones are very distinctly seen in most specimens, and totally differ in form and arrangement from those in Palæoniscus, though closely resembling the corresponding bones in Lepidotus and other Mesozoic genera. The entire opercular apparatus has an evenly rounded posterior margin; the operculum and suboperculum are large, and of nearly equal area, being divided by a line running obliquely
- ↑ "Ueber die Begrenzung und Eintheilung der Granoiden," Palæontographica, vol. xxii. 1873, p. 26.
