30 GEOLOGICAL SURVEY 
a line with the vertebral axis. So far the classification was available for identifying the aflinities 
of fossil with recent fishes, but when it was to be applied to lower divisions of the system its 
defects became hopelessly obvious. It rarely happens that fossil fishes are found so perfect as to 
retain all the fins, and when fins are absent it is not always possible to say that they never existed. 
It was in this state that Prof. Agassiz found fossil Ichthyology. Observing the constancy with 
which the scales of fossil fishes are preserved, even in the oldest rocks, when every vestige of all 
other characteristic parts have disappeared, the happy idea occurred to him of founding a classifi- 
cation upon the structure of the scales. Now it is known that the tegumentary parts of all animals 
are subject to the greatest changes and deviation from typical characters, and hence at a superficial 
view, a classification based upon the scales of fishes has an exceedingly artificial aspect. It is 
found, however, that there exists an intimate connection between the organization of the animal 
and the structure of the scales, and hence the orders founded upon that structure are perfectly 
natural groups. And nothing could afford stronger proof of the practicability of the system than 
the rapid strides that have been made in this department of Paleontology, since it was adopted. 
From a few genera and species that were before known, the number described probably amounts, 
at this time, to nearly two thousand: and what is still more important, these investigations have 
shown that no organic remains are capable of affording the Palsontologist more certain or more 
important results. For, as they extend through all the great geological periods, he has an opportu- 
nity of studying the changes that have taken place in an important class of vertebrate animals 
continued through a vast succession of ages. A class, too, that has been studied with more care in 
every stage, from the embryo up, than almost any other. 
The orders under which fishes are comprehended, according to this arrangement, are four. 
1. Placoids, fishes having scales composed of plates of enamel alone, of various sizes; some- 
times large and with pointed protuberances, and sometimes small and scattered over the body of 
the fish. This order includes the shark, ray, and indeed all the cartilaginous fishes of Cuvier, 
except the sturgeon. 
2. Ganoids, which have angular and bony scales, covered at their exposed surface with enamel, 
often elegantly sculptured ; the “gar” is a fine example of this order. 
3. Ctenoids, have scales of bone alone, without enamel, and cut, at the exposed ends, like the 
teeth of a comb; the scales of the perch exhibit these characters. 
4. Cycloids have also bony or horny scales, without enamel, more or less circular ; smooth on 
the edges, and frequently ornamented on their upper surface. The herring is a familiar example 
of this order. 
The form of these several scales is represented in 
1—Placoid. 2—Ganoid. 3—Ctenoid. 4—Cycloid. 
