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described as a sort of bird-wing fin. The naked rays, withtheir flattened surfaces, lay thick together as feathers in thewing of a bird,—so thick as to conceal the connecting mem-brane ; and fins of similar construction characterized the fa-milies of the Dipterus, Diplopterus, Glyptolepis, Cheirolepis,Holoptychius , and, I doubt not, many other families of thesame period, which await the researches of future discoverers.But the fins of another set of ichthyolites, their contempo-raries, may he described as bat-wing fins : they presented tothe water a broad expanse of membrane; and the solitaryray which survives in each was not a jointed, but a conti-nuous spear-like ray. The fins of this set, or order, arethorny fins, like those of the Acanthopterygii : the anterioredge of each, with the exception of perhaps the caudal fin,which differs in construction from the others, is composed ofa strong bony spike. Such, with some tacit reference, per-haps, to the similar Cuvierian principle of classification, werethe distinctive differences on the strength of which I usedto arrange two of my groupes of fossils on the granitic boul-der ; and the influence of the same principle, almost instinct-ively exerted,—for in writing the previous pages I scarcethought of its existence,—has, I find, given to each groupits own chapter.
Of the membranous-finned and thomy-rayed order of ich-thyolites, the Cheiracanthus, or thorny-hand (i. e. pectoral),may be regarded as an adequate representative. (See PlateVII., fig. 1.) The Cheiracanthus must have been an emi-nently handsome little fish,—slim, tapering, and describedin all its outlines, whether of the body or the fins, by grace-fully waved lines. It is, however, a rare matter to find itpresenting its original profile in the stone : none of theother ichthyolites are so frequently distorted as the Cheir-acanthus. It seems to have been more a cartilaginous andless an osseous fish than most of its contemporaries. How-