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 low^er 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 



