OF SAUROID FISHES. 211 



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 Oohte 

 formation.* In the Cretaceous formations they become 

 extremely rare.f They have not yet been discovered in, 

 any of the Tertiary strata; and in the waters of the pre- 

 sent world are reduced to the two genera^ Lepidosteus and 

 Polypterus. 



Thus we see that this family of Sauroids holds a very im- 

 portant 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 com- 

 mencement of the Chalk.. The cessation of these Reptiles 

 and of the semi-reptile Sauroid Fishes in the Tertiary for- 

 mations made room for the introduction of other predaceous 

 families, approaching more nearly to those of the present 

 creation.^ 



* The Aspidorhynchus, from the Jurassic limestone of Solenhofen, (PI 

 27*, Fig. 5,) represents the general character of the Sauroid Fishes. 



t The Macropoma is the only genus of Sauroid Fishes yet found in tho 

 Chalk of England. 



t Much light has been thrown on the history of Fishes in the Old red 

 sandstone at the base of the Carboniferous series, by the discoveries of Pro- 

 fessor Sedgwick and Mr. Murcliison, in tlie bituminous schist of Caithness, 

 (Geol. Trans. Lond. n. s. Vol. 3, part 1. ;) and those of Dr. Traile, in the 

 same schist in Orkney. Dr. Fleming also has made important observations 

 on Fishes in the old red sandstone of Fifeshire. Farther discoveries have 

 been made by Mr. Murchison of Fishes in the old red sandstone of Salop 

 and Herefordshire, The general conditions of all these Fishes accord with 

 those in the carboniferous series, but their specific details present most 

 interesting peculiarities. Many of them will be figured by Mr. MurchisoR 

 in his splendid Illustrations of the Geology of the Border Counties of Eng- 

 land and Wales. 



