216 FOSSIL FISHES. 



Fishes of the Chalk Formation. 



The next and most remarkable of all changes in the cha» 

 racter of Fishes, takes place at the commencement of the 

 Cretaceous formations. Genera of the first and second or- 

 ders (Placoidean and Ganoidean,) which had prevailed ex- 

 clusively in all formations till the termination of the Oolitic 

 series, ceased suddenly, and were replaced by genera of 

 new orders (Ctenoidean and Cycloidean,) 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 series, than to those which had preceded the forma- 

 tion of the Chalk. 



Comparing the Fishes of the Chalk with those of the elder 

 Tertiary formation of Monte Bolca, we find not one species, 

 and but few genera, that are common to both.* 



Fishes of the Tertiary Formations. 

 As soon as we enter on the Tertiary strata, another change 



* It has been already stated, that the remarkable deposite of fossil Fishes 

 at Engi, in the Canton of Claris, are referred by M. Agassiz to the lower 

 portion of the Cretaceous system. 



Many genera of these are identical with, and others closely approximate 

 to, the fishes of the Inferior chalk (Plilner kalk) of Bohemia, and of the 

 Chalk of Westphalia (secLeonhard and Bronn. Neues Jahrbuch, 1834.) Al- 

 though the mineral character of the slate of Claris presents, as we have be- 

 fore stated, an appearance of high antiquity, its age is probably the same as that 

 of the Gault, or Speeton clay of England. This alteration of character is 

 consistent with the changes that have given an air of higher antiquity than 

 belongs to them, to most of the Secondary and Tertiary formations in the 

 Alps. 



The Fishes of the Upper chalk arc best known by the numerous and 

 splendid examples discovered at Lewes by Mr. Mantcll, and figured in his 

 works. These Fishes are in an unexampled state of perfection; in the ab- 

 dominal cavities of one species (Macropoma) the stomach, and coprolites are 

 preserved entire, in their natural place. 



