=Evidence of Paleontology.=--If we had representations of all the early forms of fishes arranged in proper sequence, we could decide once for all, by evidence of paleontology, which form of fin appears first and what is the order of appearance. As to this, it is plain that we do not know the most primitive form of fin. Sharks of unknown character must have existed long before the earliest remains accessible to us. Hence the evidence of paleontology seems conflicting and uncertain. On the whole it lends most support to the fin-fold theory. In the later Devonian, a shark, _Cladoselache fyleri_, is found in which the paired fins are lappet-shaped, so formed and placed as to suggest their origin from a continuous fold of skin. In this species the dorsal fins show much the same form. Other early sharks, constituting the order of _Acanthodei_, have fins somewhat similar, but each preceded by a stiff spine, which may be formed from coalescent rays. [Illustration: FIG. 51.--_Cladoselache fyleri_ (Newberry), restored. Upper Devonian of Ohio. (After Dean.)] [Illustration: FIG. 52.--Fold-like pectoral and ventral fins of _Cladoselache fyleri_. (After Dean.)] Long after these appears another type of sharks represented by _Pleuracanthus_ and _Cladodus_, in which the pectoral fin is a jointed organ fringed with rays arranged serially in one or two rows. This form of fin has no resemblance to a fold of skin, but accords better with Gegenbaur's theory that the pectoral limb was at first a modified gill-arch. In the Coal Measures are found also teeth of sharks (_Orodontidæ_) which bear a strong resemblance to still existing forms of the family of _Heterodontidæ_, which originates in the Permian. The existing _Heterodontidæ_ have the usual specialized form of shark-fin, with three of the basal segments especially enlarged and placed side by side, the type seen in modern sharks. Whatever the primitive form of shark-fin, it may well be doubted whether any one of these three (_Cladoselache_, _Pleuracanthus_, or _Heterodontus_) actually represents it. The beginning is therefore unknown, though there is some evidence that _Cladoselache_ is actually more nearly primitive than any of the others. As we shall see, the evidence of comparative anatomy may be consistent with either of the two chief theories, while that of ontogeny or embryology is apparently inconclusive, and that of paleontology is apparently most easily reconciled with the theory of the fin-fold. [Illustration: FIG. 53.--Pectoral fin of shark, _Chiloscyllium_. (After Parker and Haswell.)]