FISHES IN CHALK FORMATION. 283 
have yet been discovered in the Cretaceous 
series ; and three or four species in the Tertiary 
formations. Among living Fishes, scales of this 
kind occur only in the two genera, Lepidosteus 
and Polypterus. 
Not a single genus of all that are found in the 
Oolitic series exists at the present time. The 
most abundant Fishes of the Wealden formation 
belong to genera that prevailed through the 
Oolitic period.*" 
Fishes of the Chalk Formation. 
The next and most remarkable of all changes in 
the character of Fishes, takes place at the com- 
mencement of the Cretaceous formations. Genera 
of the first and second orders (Placoidean and 
Ganoidian), which had prevailed exclusively in 
nil formations till the termination of the Oolitic 
series, ceased suddenly, and were replaced by 
genera of new orders (Ctenoidean and Cycloi- 
dean), then for the first time introduced. Nearly 
two-thirds of the latter also are now extinct ; but 
these approach nearer to Fishes of the tertiary 
Tories, than to those which had preceded the 
formation of the Chalk. 
that were warmer, or subject to more sudden changes of tem- 
perature, than could be endured by Fishes whose skin was 
protected only by such thin, and often disconnected coverings, as 
the membranous and horny scales of most modern Fishes. 
* The most remarkable of these are the genus Lepidotus, 
Pholidophorus, Pycnodus, and Hybodus. 
