22 THE SNAKES OF SOUTH AFRICA. 
opposite side are brought parallel with the other set, dragging up 
the hinder part of the body, and propelling the upper or anterior 
part. All this is done so smoothly and equally that the snake 
crawls with the greatest ease, and moreover is exceedingly 
graceful inits movements. A snake cannot crawl upon a perfectly 
(1) 
Fic. 12.—1. A snake crawls over the ground like this, propelled by its abdominal shields, 
the tips of which grip uneven surfaces. 
2. The erroneously popular idea of how snakes progress over the ground. 
smooth surface, such as a sheet of polished plate glass, and can 
only succeed in wriggling in a helpless sort of way. The scales 
of ‘snakes partly overlap each other. The abdominal shields can 
be partially raised by certain muscular attachments. 
THE TEETH. 
Snakes’ teeth are recurved ; that is, they are curved inwards 
towards the throat. Non-venomous snakes, with the exception 
of the blind burrowing snakes, have two rows of small teeth in 
the upper jaw, and one row in the lower jaw. The teeth are not 
set in sockets in the jaw, but are fastened to the bone by cartilage. 
If, on examination, these teeth be found to be all solid, without 
any trace of grooving, then the snake is certain to be quite non- 
venomous. But in a great number of snakes with two rows of 
upper teeth it will be found that one, two, and even three of the 
teeth in the outer row, usually about half-way back in the jaw, 
are longer than the rest, and more or less grooved. Such snakes 
should always be regarded as venomous to a greater or lesser 
degree. The Boomslang or Tree Snake (Dispholidus typus) is one 
