78 ON THE PHYSIOGNOMY OF SERPENTS. 



FORMS. 



Ophidians differ infinitely from each other relatively to 

 their size and their general form. This depends in a great 

 measure on their mode of life, the nature of the places, 

 or the element they inhabit, and also on the kind of loco- 

 motion Avhich is natural to them. The species which 

 frequent trees arc especially distinguished by their slender 

 forms ; while those Avhich prefer plains, or retire into bur- 

 rows, are recognisable ]}y their compact body, terminated 

 by a very short tail ; intermediate between these two tribes 

 as to development of parts, are a great many serpents that 

 prefer to remain on the ground always, but can climb, and 

 also swim, Avith greater or less facility ; others that delight 

 more in humid places, or never quit the water, present the 

 most varied forms, more or less suited to this species of 

 locomotion. 



A Trunk laterally compressed, is observed in the greatest 

 number of Ophidians ; but no family presents this charac- 

 ter in so marked a degree as those Tree-snakes which we 

 have included in the genus Dipsas, and Sea-serpents which 

 have the belly more or less completely keeled, in order 

 better to cut the waters. The serpents, on the other hand, 

 which merit more especially the name of terrestrial, such 

 as the Tortrix, the Calamaria, the Elaps, &c., are dis- 

 tinguished by bodies more or less cylindrical. Others, 

 such as the Tropidonotus, equally terrestrial, but excellent 

 swimmers, have a belly very broad, and rounded towards 

 the sides. Several Boas have the faculty of rendering 

 their trunk, when gliding or swimming, of the same form 

 as the Tropidonotus ; but their trunk becomes laterally 

 compressed by an opposite movement of the ribs, when 

 they prepare to climb or to roll themselves inw^ardly. 

 These changes in the size of the trunk which take place in 

 such movements are found in a greater or less degree in 

 all Ophidians, and render the exact determination of their 

 forms very difficult : for indicating that of the body we are 

 obliged to describe the figures which transverse sections 

 present, one of wiiich is made in the middle of the trunk, 



