The Cladodont Sharks. — Claypole. 367 
fused and broken skeleton as now known would be too specu- 
lative to afford a sure basis for inferences. 
But perhaps the most interesting feature, especially in a 
theoretical sense, which is presented by these Devonian sharks, 
is a peculiar flap or fold of the skin which projects horizon- 
tally in front of the caudal fin and makes a most remarkable 
showing when the hinder part of the fish is preserved. The 
caudal fin is then scarcely seen, showing merely a sharp edge 
ending in a point. But the wide expansion of the flap Justin 
front of it gives the appearance of a widening of the body 
that in a fish is, to say the least, extraordinary. Such a struc- 
ture is not perhaps quite unexampled among recent fishes, but 
in no case, so far as I know, can anything corresponding to 
the one here described be found. The flap is entirely mem- 
branous and shows no trace of fin-rays. Yet it was rather 
solid and has left at least as clear an impression on the stone 
as has the membranous margin of the pectoral fins. It can 
be looked on as a third pair of paired fins set back near the 
posterior extremity of the fish in a post-ventral position. 
They evidently may have been of great service to the fish in 
giving a powerful horizontal leverage against the water and 
so enabling it to strike in an upward or downward direction. 
They may thus have served, in a less degree, the same purpose 
as the flukes of the whale, enabling these sharks to ascend or 
to descend in the water with great ease and rapidit}'. 
These fins are attached to the sides of the animal by broad 
bases and form triangular projections whose appearance, when 
seen from below, may be likened to that of a pointed shovel. 
So far as known it seems probable that this appendage was 
common to all the species of Cladodus. 
The interest of this peculiar feature in connection with 
theoretical views that are prevalent regarding the origin of 
the paired fins is obvious. The fact of the evolution of the 
azygous fins from the primeval and embryonic continuous 
marginal membrane, seen even now in the bowfin (A mia calca) 
and other fishes and in the tadpoles, may be considered an es- 
tablished doctrine, however ditlicult it may at present be to 
account for the gradual concentration of the fin development 
at a few points in most of our recent fishes. But the parallel 
doctrine of the evolution of the paired fins — the archetypes- 
