﻿STRUCTURE OF THE PALtEONISCIDtE. 



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contact with the operculum, and in Rhabdolepis and Cosmo pdy chins also with the sub- 

 operculum; the short anterior margin is overlapped by the external suborbitals, while the 

 backwardly directed apex is continuous with the lower limb. The latter, very narrow, 

 passes downwards and slightly backwards between the posterior margin of the maxilla 

 and the interoperculum. The upper limb is especially broad and conspicuous in Nema- 

 toptychitis, Acrolepis, and others ; it is very narrow in Cosmoptychius, while in Cheirolepis 

 the lower limb is rather broader than is usually the case. This bone reminds us, in 

 many respects, much more of the large praeopercular plate of Polypterus than of the bone 

 as we find it in Teleostei and in Ganoids of more modern type. 



The gill-flap is completed inferiorly and anteriorly by a series of narrow enamelled 

 plates (dr.), representing the branchiosteyal rays, the first of which comes on immediately 

 below the interoperculum, the rest succeed placed closely along the margin of the lower jaw 

 and imbricating from behind forwards. In Palceoniscus the number of these plates is eight 

 or nine, the anterior one of each lateral series being much broader than the others, and in 

 front of these and behind the symphysis is a median lozenge-shaped plate (PI. I, fig. 6), 

 obviously the equivalent of the median "jugular" of Amia and many fossil Lepidosteoids 

 such as Dapedius, Eiignathus, &c. This median plate does not seem, however, to be present 

 in all Palceoniscidce, though obvious in Palaoniscns, Amblypterus, Elonichthys, Rhabdolepis, 

 Gonatodus. I have seen no trace of it in well-preserved specimens of Oxygnathts (PI. 

 II, fig. 3) and Cheirolepis, in which the large anterior plate of each lateral series is also 

 more triangularly oblong in shape than in the genera in which the median plate occurs. 1 



The presence of a narrow supra-temporal chain of ossicles behind the posterior 

 margin of the cranial buckler is probable, though not satisfactorily exhibited in any 

 specimen I have had the opportunity of examining. I have, however, observed, what 

 seem to me to be decided traces of their presence in Palaomscus, and in fig. 2, 

 PI. I, s. t., I have indicated them in dotted lines. The large trigonal plate [p. t.) behind 

 the skull on each side so obvious in nearly every well-preserved head, and which certainly 

 does resemble in form and position the large supra- temporal of the recent Amia, or of the 

 extinct Lepidotus and Semionotus, I must refer to the shoulder-girdle, considering it to 

 be a post-temporal or " supra-scapular." 



The orbit is completely surrounded by a circle of narrow curved ossicles (s. o.), 

 apparently four or five in number, and resembling to some extent the circumorbital 



1 In my paper on Cheirolepis already quoted, I pointed out that the plates considered by Powrie to 

 be principal jugulars were, in fact, the infraclavicular elements of the shoulder-girdle. But, by Dr. 

 Martin (op. cit.) the presence of " Kehlplatten " was accepted not merely for Cheirolepis but for 

 Palceoniscus also, in which latter genus he has actually figured them. An examination, however, of the 

 original of his fig. 8 shows a very remarkable misinterpretation of the parts exhibited. The specimen lies 

 on its side, not on its back as he evidently supposed, one of his "jugulars " being the broad portion of the 

 maxilla, while the other is part of the prceoperculum. What he has considered to be the right hyoid arch 

 is the lower jaw, and I have already referred to the fact that in the same specimen he has mistaken a small 

 portion of the palatal arch for the " sphenoideum." 



