Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/660

560 I. ovatus of W. C. Redfield, species originally referred by that author also to Palæoniscus, though he was not unaware of their essential differences from that genus, and of the likelihood of their being eventually separated. For in alluding to the stout character of the fins and their insertions, whence the specific name fultus, given by Agassiz, he says that "this character is also found to pertain in a greater or less degree to all the American species of the genus, and would perhaps warrant their separation from the Palæonisci". He notices, further, the great strength of the fulcra, their comparatively small number, and unequal length and inclination, and, as regards the tail, that the scales of the body are prolonged into the upper lobe, "but to a more limited extent than in the European species of the genus." The small extent of the gape has also been mentioned by Sir Philip Grey-Egerton.

Ischypterus was classed by Sir Philip Grey-Egerton among the Ganoidei Heterocerci (= Lepidoidei Heterocerci, Ag.) along with Palæoniscus and Amblypterus; more recently, however, it has been, by Prof. J. V. Carus, disassociated from the Palæoniscidæ and placed among the Sauroidei as remodelled by Dr. Andreas Wagner. But as I have hitherto seen no detailed account of its structure, I may here give a few particulars concerning I. latus, which will clearly show how widely this genus deviates, not only from Palæoniscus, but from the entire group of Palæoniscidæ.

In Ischypterus the body is rather deep, and strongly arcuated in front of the dorsal fin; the scales are rhomboidal and smooth; but along the middle line of the back, from the occiput to the dorsal fin, there extends a row of peculiarly shaped median scales, like those in Semionotus Bergeri, Ag., and Lepidotus minor, Ag., these being somewhat spur-shaped, with posteriorly directed points, and imbricating over each other from before backwards. They were pointed out by Mr. W. C. Redfield, who says of them that they were "sometimes mistaken for an anterior comblike dorsal." The caudal fin is comparatively short and small; it is hardly cleft, being only somewhat concave behind, and is, moreover, nearly symmetrical in external form, the upper projecting point only passing a little further back than the lower. The prolongation of the body-scales along the upper margin of the fin is very narrow and rapidly attenuating, and, although it reaches nearly to the extremity of what may be called the upper lobe, is very short, owing to the shortness of the fin itself. The rays are comparatively few in number, those of the upper lobe gradually diminishing in length towards its extremity; and the fulcra, which run along the margin of the lower lobe, are nearly as strong as the V scales, usually also called fulcra, which border the upper one above. Though this form of tail cannot be called "homocercal," inasmuch as a scaled prolongation of the body does extend nearly to the point of the upper lobe, yet, from the shortness, feebleness, and attenuation of this prolongation, along with the striking reduction of the number of