Raymond C. Osburn 173 
type of Pleuracanthus or the fin-fold type of Cladoselache, but the ques- 
tion is, from which did it evolve? In taking up this question we have 
to consider not only the type of fin but also the possibilities of the animal 
as an ancestor of modern sharks. While Cladoselache itself may not 
have been the progenitor of modern sharks, the group to which it be- 
longs is of such a generalized type that, as Dean points out (95), almost 
any later group of fishes might have arisen from it. In fact, the Clado- 
selachidxe approach more nearly to the ancestral condition of true fishes 
than any other group of which we have sufficient knowledge to form a 
judgment. This simplicity is coupled with an antiquity much greater 
than that of the Pleuracanthide. In regard to the latter family it may 
be suggested that the nuchal spine, the condition of the dermal skeleton, 
the character of the two anal fins, and the diphycercal caudal fin, remove 
it at once from our consideration as a possible ancestor of any known 
type of fish. Concerning the diphycercal caudal it is worthy of remark 
that all the oldest fishes known (Pleuropterygide, Acanthodide, and 
Diplacanthide among the sharks, the most ancient Dipnoi and Ganoidei, 
as well as the fish-like Heterostraci, Anaspida, Antiarcha, and Arthrodira, 
all older than the Pleuracanthide), had the heterocercal condition de- 
veloped, so that, as far as we are able to judge from paleontology, 
heterocercy seems to be the primitive condition in the gnathostomata. 
The diphycerey of Pleuracanthus must then have been secondarily ac- 
quired, as it seems to have been in the Dipnoi (Dollo, 92, 93). ‘There- 
fore, taking into consideration all the facts concerning these two ancient 
types of fishes, and bearing in mind that the Acanthodide, Diplacanthide, 
and Pleuropterygide (all much older than the Pleuracanthide), agree 
in having a fin-fold type of fin, we are led to believe that the evidence 
from paleontology is distinctly favorably to the fin-fold theory. It may 
be well to add here that even if the Pleuracanthus type of fin were proved 
to be primitive the branchial origin of the fins would still be unproved, 
for the fins of Pleuracanthus are no more gill-like than those of modern 
sharks. It would not at all preclude the possibility of the local origin 
of the fin structures, which is after all the most important contention of 
the fin-fold theorists. 
In regard to the embryological evidence it may be remarked broadly 
that the recent investigations tend to show that in early development 
there is scarcely a point in which the paired and unpaired fins are not in 
perfect agreement. The only important exceptions, the girdles and the 
trapezius muscle, have, I believe, a satisfactory explanation as secondary 
rather than primary characters. It was the original intention of the 
