VARIATION OF THE PIT-VIPER. 



175 



width apart. A 1, 1 and A 2, 3 now become confluent, forming 

 the dark unbroken marginal lines of the transverse band which 

 constitutes group A (text-fig. 4, a). These lines may be nearer 

 together on the back than on the sides, and are sometimes much 

 serrated. The bands, i. e. groups A, gradually become further 

 apart from each other, with the result that the markings are 

 slightly fewer in number than in the primitive type. For the 

 present I shall call this annulated form of marking Type II. It 

 constitutes a distinct branch in the direct evolution of markings 

 (text-fig. 3, a). Type III. evolves directly from the primitive 



Text-figure 4. 



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arrangement, in exactly the same way as Type II. up to the point 

 whei^e A 1 is completely divided, but distinct from A 2 and 3. 

 A 1, 1 are, however, squarish, and form a square group with A 2 

 and 3, which they equal in size. In the course of evolution A 4, 

 B 2, 3, and 4 reappear (text-fig. 4, b), and are also all of equal 

 size. In this type A 4 belongs to the dorso-lateral series, being 

 situated immediately above its position in the lateral series in 

 Type I. In the evolution from Type I. to Type III. the series 

 equalise, the dorsal narrowing and the dorso-lateral broadening. 

 The general pattern is formed, therefore, of three series of equal 



