﻿POSITION OF THE PALiEONISCIDJE. 



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ichthyology jlias still before her. As examples of Lepidosteoids may be quoted Ischypterus, 

 Lepidotus, Dapedius, Semionotus, P/iolidop/torus, Heterolepidotus, Euynathus, Pachy- 

 cormus, &c. All these fishes have, save in their angular enamelled scales, a most 

 remarkably " modern " or Teleostean-like aspect. The tail is only imperfectly heterocercal 

 (that of Ischypterus being the most heterocercal of the series ; the rays of the dorsal and 

 anal fins do not exceed in number the interspinous bones supporting them ; there are 

 well- developed ribs ; the shoulder-girdle has no infraclavicular in front of the lower end 

 of the clavicle ; the opercular bones are conformed as in Teleostei, the prgeoperculum not 

 tending to extend forwards on the cheek and having below its lower limb an inter- 

 operculum of the usual shape. 



On the other hand, in the Palaoniscida, the tail is completely heterocercal ; the rays 

 of the dorsal and anal fins greatly exceed in number the interspinous bones supporting 

 them ; there are no ribs ; the shoulder-girdle has prominent infraclaviculars in front of 

 the lower extremity of the clavicle ; the praeoperculum tends to extend on to the 

 cheek, while the interoperculum, if really present, is most abnormal in form and simulates 

 a suboperculum. 



If we now compare Palaoniscus with the living Ganoids we shall perhaps be a little 

 astonished to find that its affinities, as indicated by the skeleton, point most strongly not 

 to Lepidosteus, to which its angular scales and fulcrated fins give it a superficial 

 resemblance, but to Polyodon. In many points the resemblance is indeed so striking, 

 that if we could only clothe the sides of our Polyodon with rhombic scales and cut off its 

 long snout, it could not possibly occur to any one to class them in different groups. 



Polyodon is not, however, entirely without scales ; and those which it does possess, viz. 

 the patch of acutely lozenge-shaped scales along the side of the caudal body-prolongation, 

 with the series of V scales, or " fulcra" above (PL VII, fig. 3), are identical in form and 

 arrangement with those occurring in the same place in Palaoniscus. The internal skeleton 

 is in both constituted in a very similar type ; the notochord is persistent, there are no ribs, 

 the skeletal arrangements in the caudal region seem identical ; the rays of the dorsal 

 and anal fins are much more numerous than their supporting, interspinous bones. The 

 dorsal and anal fins of Polyodon have each two sets of interspinous bones to support them ; 

 this is also the case, at least, with the dorsal of Palaoniscus. The bones of the 

 shoulder-girdle are conformed on the very same type in both, save that the prominent 

 post-temporal of Palaoniscus seems to be wanting in Polyodon, whose supra-clavicular is 

 consequently attached above to the largely developed squamosal. In neither is the 

 operculum attached to the hyo mandibular by a joint, but secured in its place only by skin 

 and muscles. The styliform hyomandibular of Polyodon even excels that of Palaoniscus 

 in the strange obliquity of its direction, and consequently the gape shows the same enor- 

 mous width and backward extent ; the eye, too, occupies the same remarkably anterior 

 position with regard to the mouth (PI. VII, fig. 1). The snout of Palaoniscus is not, indeed, 

 developed into the enormous appendage we find in Polyodon ; nevertheless it does form 



