216 MARINE SAURIANS. 
first appeared, seems to have been the last of 
the long series, during which the oolitic and 
cretaceous groupes were in process of formation. 
In these periods the inhabitants of our planet 
seem to have been principally marine, and 
some of the largest creatures were Saurians of 
gigantic stature, many of them living in the 
sea, and controlling the excessive increase of 
the then existing tribes of fishes. 
From the lias upwards, to the commencement 
of the chalk formation, the Ichthyosauri and 
Plesiosauri were the tyrants of the ocean; and 
just at the point of time when their existence 
terminated, during the deposition of the chalk, 
the new genus Mosasaurus appears to have been 
introduced, to supply for a while their place and 
office,* being itself destined in its turn to give 
place to the Cetacea of the tertiary periods. 
As no Saurians of the present world are inha- 
bitants of the sea, and the most powerful living 
representatives of this order, viz. the Crocodiles, 
though living chiefly in water, have recourse 
to stratagem rather than speed, for the capture 
of their prey, it may not be unprofitable to 
examine the mechanical contrivances, by which 
a reptile, most nearly allied to the Monitor, was 
so constructed, as to possess the power of 
moving in the sea, with sufficient velocity to 
* Remains of the Mosasaurus have been discovered by Mr. 
Mantell in the upper chalk near Lewes, and by Dr. Morton in 
the green sand of Virginia. 
