80 CLASSIFICATION OF FOSSIL FISH. 



internally of bone, 4 and coated with enamel. The scales are 

 regularly arranged, and entirely cover the skin with an almost 

 impenetrable armour. The Sturgeons and the Lepidosteus are 

 almost the only living representatives of this order. 



II. PLACOIDIANS; from the Greek 7r\a, a broad plate. 



This order contains Fish whose skin is covered irregularly with 

 plates of dentine, often, of considerable dimensions, but sometimes 

 reduced to small points. Among existing Fish, this order com- 

 prehends only the Sharks and Rays and their allies. 



The two preceding Orders are also characterised by the un- 

 symmetrical tail ; which is now confined to the Sturgeons, the 

 Sharks and Rays, and the Lepidosteus. 



III. CTENOIDIANS ; from the Greek KTELQ (genitive Krevog), 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. CTCLOIDIANS ; from the Greek KVK\OG, 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 the majority of the existing species. 



644. Now the Fish of the oldest or Palceozoic strata belong 

 almost exclusively to the^r^ of these divisions ; and an immense 

 number of Ganoid Fish, forming several families, are exclusively 

 met with in the Old Red Sandstone formation. Many of the forms 

 presented by these are most extraordinary, being totally unlike 

 any with which we are acquainted among existing species, and 

 indicate an apparent mixture of the characters of the class of 

 Fish with those of the Crustacea. Indeed as to the real nature 

 of some of the species, 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 ; and it was not until connecting links were dis- 

 covered, that the nature of them could be certainly determined. 

 The head and body of many of these Fish were covered 

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



