INTRODUCTION. XX 
other hand, it must be confessed that it is difficult to recon- 
cile the separation of the Ophidians from the Saurians, 
according to the arrangement of Cuvier, who, to effect 
this, has been obliged to place some of these intermediate 
genera in the former, whilst he retains others in the 
latter group. Upon the whole, as I have before observed, 
it appears more consonant with nature to consider, with 
Merrem, that the whole of these three groups constitute 
but a single order. 
The movements of the Saurian reptiles are effected prin- 
cipally by means of their feet, and in some of the higher 
forms, exclusively so; but as they descend towards the 
more elongated forms of the Scinks, and other genera, in 
which these organs become more and more subordinate, 
they are greatly assisted by the lateral motion of the 
bodies, different parts of which are brought into alternate 
contact with and pressure upon either the roughness of the 
ground, or the shrubs and herbage through which they 
pass. A comparison of the progression of these, however, 
and even of the Slow-worm, with that of the true Snakes, 
will show that the latter only employ the free terminations 
of their ribs as organs of locomotion. In these the ribs 
serve absolutely and perfectly the function of feet, and 
with the exception of their being covered with integument, 
their action is exactly that of the multitudinous feet in the 
Scolopendra or Julus, the whole series of ribs coming into 
contact in succession. At the same time, it is upon the 
lateral motion of the body and the alternate lateral pres- 
sure of successive portions of it, that the Serpents mainly 
depend for the rapidity of their progression. 
It appears almost necessary to apologise for offering this 
sketch of some of the most important phenomena in the 
structure and habits of the different groups of existing 
