MOSASAURUS. 219 
movements, analogous to those of skulling. 
Although the number of caudal vertebrae was 
nearly the same as in the Monitor, the propor- 
tionate 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 further 
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 vertebrae was one hun- 
dred and thirty-three, nearly the same as in 
the Monitors, and more than double the num- 
ber of those in the Crocodiles. The ribs had 
a single head, and were round, as in the family 
of Lizards. Of the extremities, sufficient frag- 
ments have been found to prove that the Mosa- 
saurus, 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 entirely in the water, and that although 
it was of such vast proportions compared with 
