PAL^ONISCTJS, GYROLEPIS, AND PYGOPTERUS. 561 



the fin-rays of that upper lobe, it is not heterocercal to the same 

 extent as in Pcdceoniscus or Acipenser. It seems, indeed, to furnish 

 us with the first step in the transition from the typically 

 heterocercal tail to such semiheterocercal forms as in Lepidotus, 

 Dapeclius, Pholidophorus, &c, in which the body-prolongation is 

 proportionally shorter still, and the rays of the upper division of the 

 fin extend considerably beyond it. In the other fins the rays are 

 distant compared with those of Pcdceoniscus, and unarticulated for 

 the greater part of their length ; the fulcra of their anterior margins 

 are enormously large, and correspondingly few in number, being 

 totally unlike any thing we find in the Palaeoniscidae, though much 

 reminding us of those in some species of Semionotus and Lepidotus. 

 Though the notochord was probably persistent, there are obvious 

 remains of strongly developed spinous processes, interspinous bones, 

 and ribs, the latter being apparently totally absent in the 

 Palaeoniscidae. The bones of the head are, unfortunately, very badly 

 preserved in all the specimens of Ischypterus which I have had the 

 opportunity of examining ; they display, however, enough to render 

 certain the following essential points of structure. The line of the 

 top of the head slopes very rapidly from the occiput downwards and 

 forwards to the snout, which, in profile, appears rather sharp, and 

 does not form the peculiar nasal prominence over the mouth seen in 

 Pcdceoniscus and its allies. The opercular bones are conformed quite 

 according to the type characteristic of the Mesozoic Ganoids and 

 modern Teleostei. The operculum and suboperculum are rather 

 narrow ; the praeoperculum, passing first downwards, curves then 

 gently forwards, carrying the articulation of the mandible to a 

 point in front of the upper attachment of the suspensorium ; the 

 interoperculum is also distinctly visible as a small triangular plate 

 with anteriorly directed apex, and placed in front of the lower part 

 of the suboperculum and below the anterior extremity of the prae- 

 operculum. The gape is small ; the configuration" of the maxilla I 

 have not been able to determine ; but the mandible is stout and short 

 and has its dentary margin set with a row of equal-sized, small 

 and rather bluntly conical teeth. In one specimen I have seen 

 similar teeth, apparently in more than one row, in the upper part of 

 the mouth ; but, from defective preservation, it is hardly possible to 

 tell on what bone they are situated. The orbit has not the remark- 

 ably anterior position characteristic of the Palasoniscidae, but is 

 situated nearly right above the articulation of the lower jaw. 

 Beyond pretty distinct indications of frontals and parietals, and of 

 a powerfully developed parasphenoid, no further details of the 

 osteology of the head are revealed by such specimens of Ischypterus 

 as I have had at my disposal. 



The few details given above render it, however, perfectly evident 

 that the affinities of Ischypterus are not with Pcdceoniscus, but that 

 it must, on the other hand, be looked upon as the most heterocercal 

 of that great series of Lepidosteoid Ganoids especially characteristic 

 of Mesozoic times, and of which Lepidotus, Semionotus, Dapeclius, 

 PhohcJophorus, &c. arc prominent examples. To two of those. 



