VARIATION OF THE PIT-VIPKR. 



171 



many snakes. This series must not be confounded with the outer 

 ventral series, which commences on the chin-shields and is usually 

 distinctly marked throughout the body. 



The markings develop upon regular lines, and the degree of 

 this development varies on different regions of the body. That 

 on the anterior end of the body is the most primitive, as I shall 

 endeavour to show later on, but markings immediately behind 

 the head are always irregular. 



In Lachesis atrox the spots of the series on each side of the 

 body form themselves into distinct alteinating groups, which I 

 shall call A and B. Group B is less stable than A, and frequently 

 disappears altogether, either tempoiarily in the evolution of the 

 markings or permanently. 



Text-figure 2. 

 Group A. Group B. 



I \ 



On one snake the relative development of groups A and B is 

 not always constant, but it is always alike on both sides of 

 the body. 



Since group A is usually the best developed, it is this group 

 which I have counted in giving the numbers of markings in the 

 table of statistics. This number is not the same for the two sides 

 of many snakes — another reason for the faulted appeai'ance in 

 the general dorsal pattern. 



The dorsal markings of L. atrox are therefore made up on each 

 side of three longitudinal series of spots, which in turn form 

 themselves into alternating groups A and B. Each of these 

 consists of six primitive spots, which, according to their presence, 

 absence, or confluence, determine the variety of marking pro- 

 duced. In oi'der to follow their evolution and variation, I have 

 numbered those of each group in order of their stability (see 

 text- fig. 2), so that each may be referred to by a definite 

 formula. 



The ground-colour appears to be formed by the varying pro- 

 portions of a black (or dark brown) and a yellow pigment, the 

 black being the more superficial. When the shade is gi-eyish, 

 there is a smaller proportion of yellow pigment. The usual brown 

 tints are composed of a very fine powdering of the dark pigment 

 over the yellow. Quite apart from the regular markings, the scales 



