206 FOSSIL FISHES. 



Jt must be obvious that another and most important 

 branch of natural history is enhsted 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 

 application as the system now proposed. We introduce an 

 additional element into geological calculations ; we bring 

 an engine of great power, hitherto unapplied, to bear on 

 the field of our inquiry, and seem almost to add a new 

 sense to our powers of geological perception. The general 

 result is, that fossil Fishes approximate nearest to existing 

 genera and species, in the most recent Tertiary deposits; 

 and differ from them most widely in strata whose antiquity 

 is the highest; and that strata of intermediate age are 

 marked by intermediate changes of ichthyological condi- 

 tion. 



are well known, from the use made of them in rasping and polishing wood, 

 and for shagreen. 



Second Order, GANOIDJANS. (PI. 27, 3, 4. Etym. yxm, splendour, 

 from the bright surface of their enamel.) The families of this Order are 

 characterized by angular scales, composed of horny or bony plates, covered 

 with a thick plate of enamel. The bony Pike (Lepidosteus Osseus, PI. 27% 

 Fig. 1;) and Sturgeons are of this Order. It contains more than sixty ge- 

 nera, of which fifty are extinct. 



Third Order, CTENOIDIANS. (PI. 27, Figs. 5, 6. Etym. xrsve, a 

 comb.) The Ctenoidians have their scales jagged or pectinated, like the teetli 

 of a comb, on their posterior margin. They are formed of laminae 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. kvkmu 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 laminae 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 diflferent proportions during different 

 epochs; only the first two existed before the commencement of the Cretace- 

 ous 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 first two 

 Orders had become extinct. 



