170 MARINE SAURIANS. 



as in the Monitor, the proportionate length of the tail was 

 much diminished by the comparative shortness of the body 

 of each vertebra ; the effect of this variation being to give 

 strength to a shorter tail as an organ for swimming ; and a 

 rapidity of movement, which would have been unattainable 

 by the long and slender tail of the Monitor, which assists 

 that animal in climbing. There is a farther provision to give 

 strength to the tail, by the chevron bones being soldered 

 firmly to the body of each vertebra, as in fishes. 



The total number of vertebras was one hundred and 

 thirty-three, nearly the same as in the Monitors, and more 

 than double the number of those in the Crocodiles. The ribs 

 had a single head, and were round, as in the family of 

 Lizards. Of the extremities, sufficient fragments have been 

 found to prove that the Mosasaurus, instead of legs, had four 

 large paddles, resembling those of the Plesiosaurus and the 

 Whale : one great use of these was probably to assist in 

 raising the animal to the surface, in order to breathe, as it 

 apparently had not the horizontal tail, by means of which 

 the Cetacea ascend for this purpose. All these characters 

 unite to show that the Mosasaurus was adapted to live en- 

 tirely in the water, and that although it was of such vast 

 proportions compared with the living genera of these 

 families, it formed a link intermediate between the Monitors 

 and the Iguanas. However strange it may appear to find 

 its dimensions so much exceeding those of any existing 

 Lizards, or to find marine genera in the order of Saurians, 

 in which there exists at this time no species capable of 

 living in the sea ; it is scarcely less strange than the analo- 

 gous deviations in the Megalosaurus and Jguanodon, which 

 afford examples of still greater expansion of the type of the 

 Monitor and Iguana, into colossal forms adapted to move 

 upon the land. Throughout all these variations of propor- 

 tion, we trace the persistence of the same laws, which regu- 

 late the formation of living genera, and from the combina- 

 tions of perfect mechanism that have, in all times, resulted 



