THE OLD RED SANDSTONE. 81 
obtains. The fin of the Osteolepis, with its surface of enam- 
elled and minutely jointed bones, I have already described as 
a sort of bird-wing fin. The naked rays, with their flat- 
tened surfaces, lay thick together as feathers in the wing of 
a bird —so thick as to conceal the connecting membrane ; 
and fins of similar construction characterized the families 
of the Dipterus, Diplopterus, Glyptolepis, Cheirolepis, Holop- 
tychius, and, I doubt not, many other families of the same 
period, which await the researches of future discoverers. But 
the fins of another set of ichthyolites, their contemporaries, 
may be described as bat-wing fins: they presented to the 
water a broad expanse of membrane; and. the solitary ray 
which survives in each was not a jointed, but a continuous 
spear-like ray. The fins of this set, or order, are thorny-fins, 
like those of the Acanthopterygii ; the anterior edge of each, 
with the exception of, perhaps, the caudal fin, which differs in 
construction from the others, is composed of a strong, bony 
spike. Such, with some tacit reference, perhaps, to the sim- 
ilar Cuvierian principle of classification, were the distinctive 
differences, on the strength of which I used to arrange two 
of my groups of fossils on the granitic boulder; and the 
influence of the same principle, almost instinctively exerted, 
— for, in writing the previous pages, I scarce thought of its 
existence, — has, I find, given to each group its own chapter. 
Of the membranous-finned and thorny-rayed order of ich- 
thyolites, the Cheiracanthus, or thorny-hand, (¢. e€. pectoral,) 
may be regarded as an adequate representative. (See Plate 
VIL., fig. 1.) The Chetracanthus must have been an eminently 
handsome little fish — slim, tapering, and described in all its 
outlines, whether of the body or the fins, by gracefully waved 
lines. It is, however, a rare matter to find it presenting is 
original profile in the stone ; — none of the other ichthyolites 
