9 2 INTRODUCTION 



and Uropeltidae, belong to this category ; the Viperid 

 Atractaspis is also a burrowing type. 



Tree-snakes spend the greater part of their life on 

 bushes or trees. Cor alius among the Boidae, Den- 

 drophis and Dendraspis among the Colubridae, Athens 

 and various species of Lachesis among the Viperidae, 

 may be quoted as examples. 



Of Water-snakes, some are exclusively aquatic, 

 like the marine Hydrophiinse and the typical 

 Acrochordinae {Acrochordus, Chersydrus) and Homa- 

 lopsinae (Hipistes, Herpeton). Chersydrus and Hipistes 

 occur in the sea as well as in fresh water. Many 

 species of Tropidonotus (T. tessellatus and T. viperinus 

 in Europe), as well as the genera Helicops, Grayia, 

 Boidengerina, etc., among the Colubridae, Eunectes 

 among the Boidae, Ancistrodon piscivorus among the 

 Viperidae, are chiefly but not exclusively aquatic. 



Our Tropidonotus natrix stands between the 

 Ground-snakes and the Water-snakes; Boas and 

 Pythons are as much Water-snakes as Tree-snakes. 

 As shown by these and many other examples which 

 might be given, a division into categories cannot 

 always be applied with precision, nor does it convey 

 an expression of the natural relationships of the 

 species, as was believed by many systematists of the 

 last century, who appealed to such adaptations for 

 the definition of families. 



A vertical pupil denotes more or less nocturnal 

 habits. Nevertheless our European Vipers, which 



