PALiEONISCUS, GYROLEPIS, AND PYGOPTERUS. 563 



Dictyopyge, &c), as they do not show a complete heterocercy, but 

 only an indication or approximation to it " *. 



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 Eoyal 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. Liitken 

 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 Palceoniscus 

 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 Palceoniscus, 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 Palceoniscus 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 Palaeoniscidae — 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 Palaeoniscidse. 



The osteology of the head shows a still more marked deviation 

 from the type of structure in the Palaeoniscidae. The opercular 

 bones are very distinctly seen in most specimens, and totally differ 

 in form and arrangement from those in Palceoniscus, 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," Palseontographica, 

 vol.xxii. 1873, p. 26. 



