270 FOSSIL FISHES. 
It must be obvious that another and most 
important branch of natural history is enlisted 
in aid of Geology, as soon as the study of the 
character of fossil Fishes has been established on 
any footing, which admits of such general appli- 
cation as the system now proposed. We intro- 
duce an additional element into geological cal- 
culations; we bring an engine of great power, 
hitherto unapplied, to bear on the field of our 
enquiry, and seem almost to add a new sense 
to our powers of geological perception. The 
general result is, that fossil Fishes approximate 
and Sturgeons are of this Order. It contains more than sixty 
genera, of which fifty are extinct. 
Third Order, CTENOIDIANS. (PI. 27, Figs. 5, 6, Etym. 
KTEiQ, a comb.) The Ctenoidians have their scales jagged or 
pectinated, lilce the teeth of a comb, on their posterior margin. 
They are formed of laminse of horn or bone, but have no enamel. 
The Perch affords a familiar example of scales constructed on 
this principle. 
Fourth Order, CYCLOIDIANS. (PI. 27, Figs. 7, 8. Etym. 
kvkXoq, a circle.) Families of this Order have their scales 
smooth, and simple at their margin, and often ornamented with 
various figures on the upper surface: these scales are composed 
of laminse of horn or bone, but have no enamel. The Herring 
and Salmon are examples of Cycloidians. 
Each of these Orders contains both cartilaginous and bony 
Fishes: the representatives of each prevailed in different propor- 
tions during different epochs ; only the two first existed before 
the commencement of the Cretaceous formations ; the third and 
fourth Orders, which contain three-fourths of the eight thousand 
known species of living Fishes, appear for the first time in the 
Cretaceous strata, when all the preceding fossil genera of the two 
first Orders had become extinct. 
