Fossil. FISH (F OLD RKD SANDSTONE. 67 



allies ; but these form but a very insignificant part of the species 

 which are to be referred to it. 



The two preceding Orders are further characterised by the 

 peculiarity in the form of the tail, which has been pointed out 

 in the Sturgeons ( 581) ; and which, among the existing fishes, 

 is confined to that family, with the Sharks and Rays, and the 

 Lepidosteus. 



III. CTENOIDIANS ; from the Greek Krets, genitive jereros, a 

 comb. The Ctenoid fish are covered with horny or bony scales, 

 jagged like the teeth of a comb on the outer edge. The Perch 

 and many other existing genera are examples of this order, 

 which contains but few fossil forms. 



IV. CYCLOIDIANS; from the Greek KUKA.OS, a circle. The 

 Fish of this last order have their scales smooth and simple at the 

 margin, and often ornamented at the upper surface. The Her- 

 ring, Salmon, &c., are referred to the Cycloid order ; which, 

 with the preceding, includes all the existing species, with the 

 exception of the few that have been already mentioned as 

 belonging to the preceding groups. 



587. Now the Fish of the oldest or Palaeozoic strata belong 

 almost exclusively to the first of these divisions ; and of eighty 

 species of Ganoid fish, which are all that have been hitherto 

 described, upwards of fifty are exclusively met with in the 

 Old Red Sandstone formation alone. Many of the forms pre- 

 sented by these are most extraordinary, being totally unlike 

 any with which we are acquainted among existing species, and 

 indicating an obvious mixture or combination of the characters 

 of the class of Fish with those of the Crustacea. Indeed in 

 regard to the real nature of some of the species discovered a few 

 years since in Scotland, even Agassiz was at first undecided, 

 so strong was the resemblance presented by them to certain forms 

 of Crustacea, especially the Trilobites hereafter to be described 

 ( 764) ; and it was not until connecting links were discovered, 

 in which the distinctive characters of the true Fish were more 

 obvious, that the nature of the first could be certainly deter- 

 mined. The head and body of many of these fish were covered 

 by large hard plates; whilst the internal skeleton, from the 



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