Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/672

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The definition of Pygopterus given by Agassiz in the "Tableau Synoptique" is as follows:—

"A. très-allongée: D. opposée a l'intervalle entre l'A. et les V. La mâchoire supérieure déborde l'inférieure. De petits rayons le long des rayons extérieurs des nageoires."

Further on in the volume the characters of the genus are indicated more in detail. The large size of the fins, especially of the heterocercal and deeply cleft caudal, is noticed, the paired fins being less developed; also the pointed conical teeth, the comparatively small size of the rhomboidal scales, and the well-developed internal skeleton. But it is on the form of the anal fin that the greatest stress is laid:—"Mais ce qui caractérise plus particulièrement les Pygopterus, c'est qu'à cette caudale inéquilobe se joint une anale fort longue qui garnit le bord inférieur du corps sur une grande étendue." The dorsal is still stated to be placed opposite the interspace between the ventrals and the anal, but "de manière à être plus rapprochée de cette dernière.' In his description of P. mandibularis, however, Agassiz states that in it the anal is more directly opposed to the dorsal than in P. Humboldtii.

The restored outline of Pygopterus given in the Atlas to the 'Poissons Possiles' (vol. i. tab. B. fig. 3) displays, however, the same faults as the accompanying restorations of Palæoniscus, Amblypterus, &c, viz. a want of acquaintance with the structure of the head, besides considerable inaccuracies as to the general form of the body and fins. Had Agassiz been acquainted with the cranial osteology of the Palæoniscidæ, it is, indeed, impossible to conceive that on the sole ground of the possession of large laniary teeth, he could have separated Pygopterus and Acrolepis, as "Sauroïdes," from their natural allies Palæoniscus and Amblypterus, a precisely similar dentition existing, as has been subsequently shown, in several of the species which he referred to the two latter genera.

By Quenstedt a peculiarity of Pygopterus, certainly of generic value, is noticed, which seems to have escaped the attention of Agassiz, viz. the non-articulation of the principal rays of the pectoral fin. As he says:—"Die grossen ungegliederten Strahlen der Brust- flossen errinern an Pachycormus." The position of the dorsal fin is also more correctly indicated by Quenstedt, according to whom it stands, "weit hinter der Bauchflosse über der vordern Hälfte der langen Afterflosse," though he might also have mentioned that it commences in front of the latter. He also gives a figure of some of the bones of the head (operculum, maxilla, mandible, branchiostegal rays), in which the essential agreement of these in form and arrangement with the corresponding bones in Palæoniscus is clearly shown.

The peculiar form of the anal fin is also emphasized by Germar, by whom the position of the dorsal fin is correctly stated in the following words:—"Man erkennt diesen Fisch sehr leicht an den