Page:Geology and Mineralogy considered with reference to Natural Theology, 1837, volume 1.djvu/215

 Rh Magnesian limestone, forming about one-fifth of the total number yet observed in this formation. Very large bones of this voracious family occur in the lias of Whitby and Lyme Regis, and its genera abound throughout the Oolite formation. In the Cretaceous formations they become extremely rare. They have not yet been discovered in any of the Tertiary strata; and in the waters of the present world are reduced to the two genera, Lepidosteus and Polypterus.

Thus we see that this family of Sauroids holds a very important place in the history of fossil Fishes. In the waters of the Transition period, the Sauroids and Sharks constituted the chief voracious forms, destined to fulfil the important office of checking excessive increase of the inferior families. In the Secondary strata, this office was largely shared by Ichthyosauri and other marine Saurians, until the commencement of the Chalk. The cessation of these Reptiles and of the semi-reptile Sauroid Fishes in the Tertiary formations made room for the introduction of other predaceous families, approaching more nearly to those of the present creation.