~~ 
THE OLD RED SANDSTONE 7a 
The fins are quite a study. I have alluded to the connect- 
ing membrane. In existing fish this membrane is the princi- 
pal agent in propelling the creature; it strikes against the 
water, as the membrane of the bat’s wing strikes against the 
air; and the internal skeleton serves but to support and stiffen 
it for this purpose. But in the fin of the Osteolepis, as in 
those of many of its contemporaries, we find the condition 
reversed. The rays were so numerous, and lay so thickly, 
side by side, like feathers in the wing of a bird, that they pre- 
sented to the water a surface of bone, and the continuous mem- 
brane only served to support and bind them together. In the 
fins of existing fish we find a sort of bat-wing construction ; in 
those of the Osteolepis a sort of bird-wing construction. The 
rays, to give flexibility to the organ which they compose, 
were all jointed, as in the soft-finned fish — as in the her- 
ring, salmon, and cod, for example; and we find in all the 
fins the anterior ray rising from the body in the form of an 
angular scale: it is a strong, bony scale in one of its joints, 
and a bony ray in the rest. The characteristic is a curious 
one. 
It is again necessary, in pursuing our description, to refer 
for illustration to the purely cartilaginous fishes. In at least 
all the higher orders of these, furnished with movable jaws, 
such as the sturgeon, the ray, and the shark, the mouth is 
placed far below the snout. The dog-fish and thorn-back are 
familiar instances. Further, the mouth in bony fishes is 
movable on both the upper and under side, like the beak of 
the parrot; in the higher cartilaginous fishes it is movable, as 
in quadrupeds, on the under side only. In all their orders, 
too, except in that of the sturgeon, the gills open to the water 
by detached spiracles, or breathing-holes; but in the stur- 
geon, as in the osseous fishes, there is a continuous linear 
