768 THE AMERICAN NATURALIST. | [Vor. XXXVI. 
many species is of greater value than an examination of few 
specimens of few species, we must, I believe, conclude that the 
doctrine of the origin of the paired limbs from lateral dermal 
folds rests upon strong foundations. Reducing to simplest 
terms our knowledge of the earliest sharks or shark-like fishes, 
among which the ancestral jaw-bearing vertebrates are reason- 
ably sought, we find that of forty odd species, representing 
fifteen genera (Paleozoic), whose fin characters are known, all 
but those of a single genus, and possibly a second, are dis- 
tinctly in favor of the lateral-fold theory. And the exceptions 
to the rule are the most recent forms and in important struc- 
tural regards the least primitive. 
More concretely, such genera as the Upper Silurian and 
Lower Devonian Parexus (two species), Climatius (eight spe- 
cies), Diplacanthus (five species) (=family Diplacanthide), 
Cheiracanthus (three species) (Acanthodida), and the long- 
enduring Acanthodes (twelve species) (Lower Devonian to 
Permian); as the Upper Carboniferous Acanthodopsis (but we 
may put the last aside, since its fin structures are not as well 
known as in the neighboring forms); as the Permian Prota- 
canthodes and Traquairia, — all of these ancient sharks have 
their paired limbs in the form of distinct longitudinal skin folds, 
and these paired fins, furthermore, are essentially similar to the 
unpaired ones. This conclusion is based upon numerous and 
well-preserved specimens. The next ancient sharks whose fin 
structures are described are the (now accepted as) Upper 
Devonian cladoselachids (about two genera and six species). 
These have the paired fins extended along the body, their 
hinder web narrowing gradually and merging with the body 
wall, showing no trace of a lobate fin base projecting from the 
body. Here, too, the paired fins closely resemble the unpaired 
fins. Structural details in these forms are known from a score 
or more of well-preserved specimens. A kindred Upper Devo- 
. nian shark is Ctenacanthus, whose fin structures have hitherto 


been unknown; but an interesting specimen of this form, show- 
ing pectoral ám and spine zz 5274, has now been discovered, and 
_ it shows convincingly that this shark had a fin-fold type of fin. 
he radials pass to the margin of the fin, as in Cladoselache, 
