86 THE OLD RED SANDSTONE. 
and Pterichthys, may throw some light on the apparently 
membranous condition of fin peculiar to the families of this 
order. What appears in the fossil a mere scaly membrane at- 
tached to a single spine of bone, may have had in the living 
animal a cartilaginous framework, like the fins of the dog- 
fish and thorn-back, that are amply furnished with rays of carti- 
lage — though, of course, all such rays must have disappearec 
in the stone, like the rest of the internal skeleton. Unques- 
tionably, the caudal fin of the two last described fossils must 
have been strengthened by some such internal framework ; 
for, as they differ from the other fins, in being unprovided 
with osseous spines, they would have formed, without an in- 
ternal skeleton, mere pendulous attachments, altogether unfit- 
ted to serve the purposes of instruments of motion. There 
may be found in the bony spines of all this order direct proof 
that, had there been an internal skeleton of bone, it would 
have survived. The spines run deep into the body, as a 
ship’s masts run deep into her hulk; and we can see them 
standing up among the scales to their termination, in such 
bold relief, that, from a sort of pictorial illusion, they seem 
as if fixed to the creature’s sides, and foreshortened, instead 
of rising in profile from its back or belly. (See Plate VIIL, 
fig. 1.) The observer will of course remember, that, in the 
living animal, the view of the spine must have terminated 
with the line of the profile, just as the view of a vessel’s mast 
terminates with the deck, though the mast itself penetrates to 
the interior keel. Now, it must be deemed equally obvious, 
that, had the vertebral column been.of bone, not of cartilage, 
instead of exhibiting no trace, even the faintest, of having 
ever existed, it would have stood out in as high relief as the 
internal buts or stocks of the spines. And such are the 
general characteristics of a few of the ichthyolites of this 
