FISHES. 285 



in the oolitic period, the different stages of which contain 

 thirty -five genera of this order. They greatly decrease in 

 numbers in the cretaceous and tertiary strata, and have a 

 few representatives in our seas. "We conclude, therefore, 

 that with the close of the oolitic period, the ganoids com- 

 menced their decline. 



The Cycloids, which include the carp and the pike, present 

 a remarkable contrast to the placoids and ganoids ; for none 

 of the fossil fishes found in the palaeozoic, triasic, or oolitic 

 strata, belong to the cycloid order. Their creation dates 

 from the commencement of the newer stages of the creta- 

 ceous period. They were introduced into the seas that 

 deposited the upper beds of the white flinty chalk. They 

 increased in numbers in the tertiary strata, and have 

 attained their maximum development in our modern seas. 

 All the fossil genera appertain to families now living. 



The Ctenoids followed the cycloids in their geological 

 march. They commenced their career with a few genera in 

 the upper stage of the chalk. A great many genera are 

 found in the tertiary strata, but their maximum development 

 has taken place in the modern epoch. Like the cycloids, all 

 the fossil genera belong to existing families. 



From this comparative view of the four orders of fishes, 

 we learn that the placoids and ganoids had their greatest 

 development in the seas of the past, whilst the cycloids and 

 ctenoids have their maximum development in the waters of 

 the present epoch. Four-fifths of the fishes now living 

 belong to the cycloid and ctenoid orders ; the remaining 

 fifth consists of placoids and a limited number of ganoids. 

 Unlike the mollusca, whose generic forms have lived through 

 many successive periods, the different faunas of fishes are 

 separated from each other by well-defined genera. Each type 

 appears to have been destined to exist for a limited time, as 

 the entire creation of one epoch differs from that of another. 



Thus no fossil fish obtained from strata anterior to the 

 chalk is identical with any genus now living. All these 

 ancient fishes differ from those of our modern seas by well- 

 defined structural characters. 



The number of extinct genera is very considerable even 

 in strata that are regarded (speaking comparatively) a? 

 modern. Thus the fishes of Grlarus and Monte-Bolca, 



