﻿STRUCTURE OF THE PAL/EONISCIDvE. 



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beni, from Mansfeld, made out what he considers to be the duplex vomer (op. cit., pi. xxii, 

 fig. 11 v). An examination of the original specimen in the Museum at Gottingen has, 

 however, failed to convince me of the correctness of his determination, though indeed it is 

 extremely probable that a double vomer was present in the Paleeoniscida. 



The hyomandibular in Palaoniscus (PI. I, fig. 3, Ji. m.) is an elongated rather slender 

 bone, which descends obliquely downwards and backwards from the squamosal region of 

 the skull to the neighbourhood of the quadrate articulation, and is slightly bent a little 

 below the middle, the posteriorly directed angle corresponding with the junction of the 

 operculum and interoperculum. Its form is in the main cylindrical, though it is a little 

 expanded and laterally flattened at the angle, above which it enlarges very gradually 

 towards the upper extremity, becoming also a little flattened antero-posteriorly. The 

 upper or longer portion is very obliquely placed on the side of the head, so that its 

 posterior margin, in contact Avith the operculum, looks more upwards than backwards, the 

 opposite anterior margin coming, in like manner, to look so much downwards as to be 

 nearly in contact with the upper edge of the posterior part of the palato-quadrate arch. 

 The lower and shorter portion of the bone below the angle approximates more to the 

 vertical, and is in relation with the interoperculum behind and to the posterior margin of 

 the palato-quadrate arch in front. Externally the hyomandibular was covered and 

 concealed by the prse-operculum, though, owing to displacement by crushing, a portion 

 of it is usually seen between the last-named bone and the operculum. 



This bone was evidently cartilaginous at both extremities, and, to judge from its 

 hollowness, the cartilage must have extended from above and below a long way into its 

 interior. I have seen no trace of a symplectic ; this element may have been absent as in 

 Polypterm, or completely unossified as in Polyodon. The configuration of the hyomandi- 

 bular is essentially the same in every genus of Palaoniscida in which I have succeeded 

 in obtaining a view of it, as in Nematoptychius, Oxygnathus (PI. II, fig. 2), ElonichtJiys, 

 Cycloptychius, &c. In Amblypterus its position is much less oblique, and the gape 

 consequently smaller in proportion (PI. II, fig. 1). 



Extending from the articulation of the lower jaw, and the lower part of the hyoman- 

 dibular, forwards to the prefrontal region, is a somewhat narrow and elongated bony 

 lamina, representing the hard palate or palato-quadrate arch (PI. I, fig. 3, pa.). The 

 position of the posterior and broader part of this lamina is nearly vertical ; externally it is 

 concave or longitudinally hollowed, forming a wide shallow groove covered externally by the 

 broad posterior part of the maxilla in which the levator muscle of the lower jaw must have 

 passed, as in the recent Polyodon (PI. VII, figs. 1 and 2), nearly horizontally backwards, 

 turning round behind, as it also does in that recent form, at a very sharp angle to attach 

 itself to the coronoid part of the lower jaw. Immediately behind the orbit the upper 

 margin of the palato-quadrate lamina is prominently angulated, and here its plane becomes 

 also a little twisted so that the narrower part passing on below the eye has its surfaces 

 looking more upwards and downwards. The greater part of this palato-quadrate lamina 

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