16 INTRODUCTION 



species or individuals some of the subcaudals are 

 single, and the others paired. When the number of 

 subcaudals is given in the descriptions, each pair is 

 reckoned as one, and the conical or spine-like shield 

 which caps the end of the tail is not included. These 

 numbers afford important characters for the definition 

 of species, and sometimes also for the distinction of 

 sexes. The subcaudals are nearly always much fewer 

 than the ventrals, but the difference is often not so 

 great in the males as in the females, the tail of 

 which is usually shorter in proportion to the body. It 

 is noteworthy that in many species, if the number of 

 subcaudals (C.) be added to that of the ventrals (V.), 

 the total is nearly the same in the male as in the 

 female, however much the respective numbers may 

 differ when taken separately. The following figures 

 may be given by way of example, taken from British 

 specimens : 



Coronella austriaca : S V. 154; C. 58=212 



? V. 165; C. 48 = 213 



Vipera herns: $ V. 138; C. 35 = 173 



? V. 144; C. 29=173 



>» f> 



Although this rule is by no means universal, and 

 does not apply at all to some species, it will be found 

 to hold good in many cases, and is of interest in 

 showing that the changes that have taken place in 

 the vertebral column (the vertebrae corresponding in 

 number to the shields), according to the sexes, have 



