Devonian Era in Ohio Basin. — Claypole. 349 
The dermal covering of the cladodonts consisted chiefly of shagreen 
of very minute pattern which is constantly and abundantly preserved. 
But on the ventral surfa'ce the skin is covered with fine, thin, spirally 
arranged scales strongly suggesting and simulating the armour of a 
tile-scaled ganoid. 
Behind the ventral fins is a remarkable organ, it' such it can be called, 
consisting of a flap or fold of the skin extending horizontally on each 
side of the body just in front of the caudal fin. It is semicircular, at- 
tached by the straight side to the body. It contains no rays, but is 
entirely composed of soft membranous tissue. Its presence gives to 
the hind end or the body viewed from above or below a most peculiar 
appearance.. The sharp termination of the body ends in a point and 
together with the folds just described presents the form of a shovel. 
Front the side the full outline of the caudal fin is seen and merely the 
thin edge of these fin-folds is visible. 
The peculiar organ, last mentioned, may prove of no little 
interest in connection with certain theoretical views that are 
prevalent regarding the outline of the paired fins. The evolu- 
tion of the azygous fins from the primaeval and continuous 
marginal membrane seen yet in the bow-fin (Amia calva) and 
some other fishes and in the tadpoles may be considered an es- 
tablished doctrine however difficult it may at present be to ac- 
count for the gradual concentration of fin-development at a few 
points in our recent fishes and to understand the fusion of the 
basals and their removal over a great part of the original line. 
But the parallel doctrine of the evolution of the paired fins — 
the archetypes of the four limbs of the higher vertebrates — 
from a supposed similar but horizontal continuous fold does 
not at present rest on an equally secure basis. If it is true, 
then changes have occurred in these more profound than any 
nf which we have evidence in the case of the former (azygous) 
organs. And the greater amount of supposition required in 
the case of the lateral fin-fold in consequence of the absence 
or scarcity of intermediate forms is not to be lightly estimated. 
From this point of view therefore, it may be exceedingly in- 
teresting to find what may prove to be an ancient relic of such 
a primaevel lateral fin in these Devonian elasmobranchs. If 
such an archetypal fold extended from the pectoral to the ven- 
tral fins it may well have extended farther to the place of these 
post-ventrals, if we may so call them, and then have run into 
the caudal and fused with the dorso-vcntral fold, thus en- 
compassing the body laterally as the other encompassed it 
vertically. 
