THE ORDER OF SNAKES—OPHIDIA. 
THE principal characteristic of this order is an exceedingly elongate body, 
covered with scales, and either destitute of limbs, or provided with very 
rudimentary ones, in the form of a spur on each side of the vent. 
Snakes propel themselves by lateral undulations, with the aid of 
sharp-edged abdominal plates, and loosely articulated ribs moving with the 
vertebral column, as thus described by Giinther :—* When a part of their 
body has found some projection of the ground which affords it some 
point of support, the ribs, alternately of one and the other side, are 
drawn more closely together, thereby producing alternate bends of the 
body on the corresponding side. The hinder portion of the body being 
drawn after, some part of it finds another support on the rough ground 
or a projection; and, the anterior bends being stretched in a straight 
line, the front part of the body is propelled in consequence. During 
this peculiar kind of locomotion, the broad shields of the belly are of 
great advantage, as by means of the free edges of those shields they are 
enabled to catch the smallest projections on the ground, which may be 
used as points of support. A pair of ribs correspond to each of these 
ventral shields. The snakes are not able to move over a perfectly smooth 
surface.” 
The number of abdominal plates generally corresponds to the 
number of vertebre, the ribs being double that number. These plates 
are of some value as a distinguishing characteristic between venomous 
and non-venomous snakes. Thus (speaking of Australian species) snakes 
with more than 240 abdominal plates, are generally not venomous, as 
so large a number has never been noticed in any other than the Boa or 
Python tribe, and some of the Tree Snakes of the genus Dipsas. They 
have neither breastbone or sternum nor eyelids, and both jaws and the 
palate are usually toothed ; the mandibles, which are united in front by 
an elastic ligament, are generally very extensible. 
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