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,[1] being itself destined in its turn to give place to the Cetacea of the tertiary periods. As no Saurians of the present world are inhabitants 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 overtake and capture such large and powerful fishes, as from the enormous size of its teeth and jaws, we may conclude it was intended to devour.
The head and teeth, (Pl. 20.) point out the near relations of this animal to the Monitors; and the proportions maintained throughout all the other parts of the skeleton, warrant the conclusion that this monstrous Monitor of the ancient deep was five and twenty feet in length, although the longest of its modern conveners does not exceed five feet. The head here represented measures four feet in length, that of the largest Monitor does not exceed five inches. The most skilful Anatomist would be at a loss to devise a series of modifications, by which a Monitor could be enlarged to the length and bulk of a Grampus,[2] and at the same time be fitted to move with strength and rapidity through the waters of the sea; yet in the fossil before us, we shall find the genuine characters of a Monitor maintained throughout the
