Page:Geology and Mineralogy considered with reference to Natural Theology, 1837, volume 1.djvu/220

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The next and most remarkable of all changes in the character of Fishes, takes place at the commencement of the Cretaceous formations. Genera of the first and second orders (Placoidean and Ganoidian,) which had prevailed exclusively 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 formation 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.

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