VARIATION OF THE PIT- VIPER. 



169 



therefore so inconstant, and so completely connected by inter- 

 mediate forms, that they cannot in themselves be considered 

 sufficient for the distinction of species. 



III. System of Markings. 



Before discussing the evolution of markings and their individual 

 variation, I will outline the general system of arrangement of 

 markings common to fill colour varieties of L. atrox. As a working 

 hypothesis on this subject, Mr. Boulenger drew my attention to 

 Dr. Zenneck's paper " Die Zeichnung der Boiden "*, and I have 

 found that his theory — that the dorsal and lateral markings of 

 Boids are made up of four paired longitudinal series f — is equally 

 applicable in the case of L. atrox. Of these, only three are 

 usually present, but in some specimens the fourth is present upon 

 the head. It is the constant relationship between the spots 

 of the series of one side of the snake which forms the regular 

 pattern, but those of one side are quite independent of those 

 on the other, thus accounting for the asymmetrical appearance 

 of the markings on the median dorsal line, which is sometimes 

 very marked J. 



When present on the head, the first or dorsal series commences 

 as a streak, or a few broken spots. Occasionally it forms a 

 A-shaped marking with its fellow. On the body this series is 

 usually the broadest and most marked, and, in individuals 

 where it tends to disappear, it is always the last to go. The 

 dorso-lateral or second series, when present on the head, may 

 either consist of an oblique narrow streak or of broken spots of 

 this streak, originating above the ocular shield, or of a cross-bar or 

 blotch in front of the eyes. This second series is rather unstable 

 on the body, for though it may be of equal development to the 

 dorsal, it is more often hardly discernible and certain spots 

 frequently become confluent with their neighbours of the first 

 series. In some forms the markings of these two paired series 

 form a single dark triangular area on the head, but are more 

 often irregular and indistinct. 



The lateral series is always present on the head of those snakes 

 exhibiting dorsal markings. It forms a broad black streak from 

 the posterior border of the eye to the commissure of the jaws, 

 usually passing through the sixth and seventh labials. Rays 

 proceeding from it may be present on the other upper labials, or 

 may take the form of isolated spots. The post-ocular streak is 

 always separated from the superciliary streak by a narrow area 

 of the lighter ground-colour, and is sometimes also outlined in 



* Tubing en Zool. Arb. iii. (1898). 



f These four series are termed Dorsal (D.), Dorso-lateral (D.L.), Lateral (L.), and 

 Ventro-lateral (V.L.) by Mr. Boulenger, 'Snakes of Europe' (1913). p. 30. This 

 lettering is followed in my text-fig. 1. 



X Mr. Boulenger has drawn attention to this bi-lateral asymmetry in his op. cit. 

 p. 33, and he mentions a specimen of Lachesis alternatus with 24 markings on the 

 left side and 27 on the right. 



