﻿STRUCTURE OF THE PALJEONISCHLE. 



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length of the jaw ; it is broadest behind, tapering gradually towards the symphysis, a little 

 behind which the centre of ossification is placed. In Cheirolepis, the lower margin shows 

 in front, and below the ossific centre, a wide, shallow notch. The posterior-superior angle 

 is produced a little backwards as a sharp, posteriorly directed process, below which the 

 thin and short posterior margin is rounded. The inner surface is concave, conspicuously 

 grooved in front, where the bone is thickest, for the anterior part of the Meckelian 

 cartilage. The superior tooth-bearing margin is a little reflected inwards, as in the 

 corresponding part of the maxilla. The dentary overlaps behind a well-marked angular 

 (PI. I, fig. 2, ay.), forming the very obtusely rounded posterior-inferior angle of the jaw. 

 Internally, the Meckelian cartilage was covered by a splenial element (sp.), which in 

 general form resembles that of Polyodon, though more largely developed ; its upper 

 margin is often seen to have teeth, as in Palceoniscus (PI. I, figs. 3 and 4). In Nema- 

 topty chins (PI. I, fig. 10) it is of a somewhat lanceolate shape, rounded behind, acutely 

 tapering in front ; the upper margin is tolerably straight, the lower convex. The 

 articular element (ar.) is rarely distinctly seen ; it bears on its upper aspect a rounded exca- 

 vation or notch for articulation with the quadrate, as is well exhibited in the figure of 

 the head of Oxygnathus given in PI. II, fig. 2. This articular notch has also been 

 represented by Messsrs. Hancock and Atthey in Elonichthys Egertoni} 



The only element of the hyoid of which I have seen any trace is the ceratohyal, a 

 tolerably slender bone, slightly expanded at the extremities, narrower in the middle. I 

 have seen the ceratohyal clearly enough in more than one specimen of Oxygnathus, also 

 in Elonichthys, and in these cases its position has been parallel to the lower jaw, within 

 and covered by the branchiostegal plates. Possibly the ceratohyal was, as in the recent 

 Polyodon, the only element of the hyoid apparatus ossified. 



There are no bones more readily distinguishable in the heads of the Palceoniscidce 

 than those of the opercular apparatus, and yet there is room for some difference of opinion 

 as to the names which some of the pieces should bear, if we aim at finding a complete 

 morphological correspondence between them and the opercular elements in the Teleostei 

 and in the more modern type of Ganoids (Lepidosteoidei and Amioidei). Regarding 

 the operculum there can, however, be no dispute. This is, in Palaoniscus (PI. I, fig. 2, 

 op) an oblong plate, broader below than above, and very obliquely placed on the side 

 of the head, owing to the peculiar direction of the suspensorium. Of its angles the 

 anterior-superior one is very acute, the posterior-inferior less so, in fact nearly a right 

 angle ; the anterior-inferior angle is rounded, but the posterior-superior is so little marked 

 that the superior and posterior margins curve round nearly uninterruptedly into each 



1 'Ann. and Mag. Nat. Hist.' (4), i, pi. xv, fig. 3. The lower jaw of Palceoniscus is described by 

 Martin (op. cit.) as consisting only of two parts, " articulare " and " dentale," the former being repre- 

 sented by him in his fig. 5 and in his restored diagram (fig. a) as enormously larger than it really is. 

 However, an examination of the original of his fig. v, a head crushed from above, betrays the fact that he 

 has mistaken the broad posterior portion of the maxilla for the " articulare," while his "dentale" is a part 

 of the palatal arch. 



