284 PALAEONTOLOGY. 



points of the denticulations render the scaly surface of the fish 

 rough to the touch (fig. 186). The skeleton is osseous, and 

 the Perch is typical of the order. The ctenoids were created 

 towards the close of the cretaceous period. Their remains 

 are found in the upper stages of the chalk of England and 

 Germany. The schists of Glarus contain great numbers. 

 They are abundant in the strata of Monte-Bolca, the London 

 clay, and the Parisian tertiaries. The gypsums of Aix and 

 the fresh-water strata of (Eningen contain numerous species. 

 Almost all the genera of this order found in the cretaceous 

 rocks are extinct, and more than half the genera found at 

 Monte-Bolca are so likewise, whilst in the upper stages of 

 the tertiary period the fossil species belong to extinct forms 

 of existing genera.* 



The profound researches of Professor Agassiz on fossil 

 fishes have enabled that learned zoologist to deduce certain 

 important generalisations from their palaeontological history, 

 a condensation of which we shall endeavour to embody in our 

 resume of the class. 



The Placoids, among which are the rays and the sharks, 

 were created at the commencement of the palaeozoic epoch, 

 and were introduced into the seas that deposited the Silurian 

 rocks. They attained the maximum of their generic develop 

 inent in the carboniferous stage. They diminished in the 

 triasic, were again augmented in the oolitic, and remained 

 stationary to the close of the tertiary epoch. The rays and 

 the sharks are the only representatives of this group in our 

 modern seas. We conclude, therefore, that from the close 

 of the palaeozoic or first great epoch of the animalisation 

 of our globe, down to the present time, the placoid fishes 

 have exhibited an order of decadence, both as regards the 

 number of the families and the genera they contain. 



The Ganoids, which include the Polypterus and Lepidosteus, 

 have obeyed a law of development nearly akin to that which 

 regulated the decline and fall of the placoids. The palaeozoic 

 rocks contain thirty-four genera of ganoids, of which twenty- 

 eight belong to the Devonian stage. They dimmish in 

 numbers in the triasic, and attain their maximum development 



* As the Cycloid and Ctenoid orders contain no extinct families, we Lave 

 omitted the description thereof; for which consult " Cuvier's Rgne Animal," 

 find " Agassiz's Poissons Fossiles. 11 



