DOUBLE-WALKERS. 157 



hinder parts of the body and the tail greatly com- 

 pressed, so as to form a vertical oar. Many, if 

 not all, of the terrestrial species, however, are 

 capable of swimming, and will take to the water 

 of their own accord. 



The geographical distribution of the Ophidia 

 is very extensive : they are spread over the whole 

 torrid and temperate regions of the globe. With 

 the exception of the Samoa group, the islands 

 of Polynesia are, however, destitute of them. 

 The Order is divided into five Families, Am- 

 pliisb(^7iadce, Boadcs, Coluhridcs, Viperadcs, and 

 HydrophidcB. 



Family I. AiviPHiSBiENAD^. 



{Douhle-ivalkers.) 



There are found in this Family of Serpents 

 several traces of structure which belong rather to 

 the preceding Order than to the present. Their 

 food consists for the most part of ants and other 

 small insects, the nature of which does not re- 

 quire that these Serpents should possess the 

 power of dilating the mouth and throat, which we 

 have described as common to the Order ; hence 

 the upper jaw is fixed to the skull and the inter- 

 maxillary bones, as in the preceding Orders of 

 reptiles, and in the higher animals ; while the 

 lower jaw is jointed directly with the skull, and 

 its two branches are soldered together in front. 

 The bony frame of the orbit of the eye is incom- 

 plete behind, as we observed it in the Geckotidce ; 

 and the eye is so minute as in most cases to be 

 with difiiculty discernible in the adult animal ; 



