184 MARINE SAURIANS. 
Mr. Conybeare remarks, with his usual acu- 
men, that *' the reasons of this variation from 
the proportions of the posterior extremities of 
quadrupeds in general, are the same which lead 
to a similar diminution of the analogous parts in 
Seals, and their total disappearance in the ceta- 
cea, namely, the necessity of placing the centre 
of the organs of motion, when acting laterally, 
before the centre of gravity. .For the same 
reason, the wings of birds are placed in the fore 
part of their body, and the centre of the moving 
forces given to ships by their sails, and to steam- 
boats by their paddles, is similarly placed. 
The great organ of motion in fishes, the tail, is 
indeed posteriorly placed, but this by its mode of 
action generates a vis a tergo, which impels the 
animal straight forwards, and does not therefore 
operate under the same conditions with organs 
laterally applied." G. T. V. 5, p. 579. 
I shall conclude this detailed review of the 
peculiarities of one of the most curious, as well 
as the most ancient, among the many genera 
of extinct reptiles presented to us by Geology, 
with a few remarks on the final causes of those 
deviations from the normal structure of its pro- 
per type, the Lizard ; under which the Ichthy- 
osaurus combines in itself the additional charac- 
ters of the fish, the Whale, and Ornithorhynchus. 
As the form of vertebrae by which it is associ- 
ated with the class of fishes, seems to have been 
