FORMS. 
79 
and a second near the base of the tail ; whence it results 
that serpents with compressed bodies present an oval sec- 
tion more or less elongated, which in the Hydrophis ap- 
proaches to a lanceolate figure ; in serpents entirely ter- 
restrial, or with a cylindrical body, the section is more or 
less orbicular : in the swimmers, or those with large and 
convex bellies, the back assuming somewhat of the keeled 
shape, the section has a triangular form, or that of a penta- 
gon, with the angles much rounded. This last figure is 
observed in the most perfect degree in serpents in which 
the lower part, more or less flattened, is separated at the 
flanks by an obtuse angle : this is termed an angular ab- 
domen. This form is particularly observed in many of the 
climbing snakes ; several Dendrophis even have the lateral 
edges of the belly furnished with a salient angle in the 
form of a keel. 
The form of the tail is still more various than that of 
the trunk : this organ also has very different functions to 
perform. The tail of burrowing snakes, excessively com- 
pact and short, of equal thickness and conical at the point, 
serves to second and direct the movements of the trunk, 
and perhaps to dig into the earth. In the greatest num- 
ber of terrestrial snakes it is a little longer, but very vigor- 
ous and conical, offering a solid fulcrum for the body, of 
which it sustains the whole weight, when the animal rears 
itself erect and stiffens itself like a stick. To fulfil the 
functions of an oar and a rudder, it is flattened in a verti- 
cal direction, and is short and lanceolate, in sea-serpents ; 
but this form is not absolutely necessary for locomotion 
in water ; for several other aquatic Ophidians have their 
tail of the ordinary form. When this member is long and 
slender, as in tree- snakes, it acquires, besides its other pro- 
perties, the faculty of entwining itself around branches, and 
of capturing or twisting in its folds the animals on which 
these serpents feed. Yet a prehensile tail, in the strict 
meaning of the term, that is to say, one which possesses 
the faculty of rolling itself completely inwards, is only 
found in the Boas ; the shorter it becomes, the more fitted 
it is to fix itself to any object, provided that it can embrace 
it ; it is then sufficiently vigorous to support the whole 
