Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/661

Rh the fin-rays of that upper lobe, it is not heterocercal to the same extent as in Palæoniscus 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, Dapedius, 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 Palæoniscus, 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 Palæoniscidæ, 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 Palæoniscidæ. 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 Palæoniscus 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 præ- 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 remarkably anterior position characteristic of the Palæoniscidæ, 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 Palæoniscus, 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, Dapedius, Pholidophorus, &c. are prominent examples. To two of those,