ox THE DORSAL SCALE-ROWS IN BRITLSH SNAKES. 357 



23. On the Variation in the Number of Dorsal Scale-rows 

 in our British Snakes. J3y Miss Joan B. Procter, 

 F.Z.S. 



[Received August U, 1919 : Read November 4,, 1919.] 



(Text-figures 1-3.) 



In a valuable paper * Dr. Ruthven has drawn attention to 

 certain points in the lepiclosis of the Garter-snakes or North 

 American Trojndonottts of the group Thamno2)his, and has for- 

 mulated a series of laws regarding the loss of cei'tain row.-s of 

 dorsal scales, which throw much light on phylogenetic questions. 

 The priucipal of these are as follows : — 



" That the decrease in the number of scale-rov:s posteriorly is 

 brought about in all of the forms of garter-snakes by the loss of 

 certain definite rows. 



" That the order in lohich, tltese rows are lost 'posteriorly in the 

 different forms is the same as in the form haviny the maximum 

 number of roios for the genus. 



'■'•The individual, geographic^ and racial variations in the number 

 of dorsal scale-rows in the garter-snakes is brought about by the 

 shortening and loss of the same scale-rotvs as are ordinarily dropped 

 posteriorly in conformity loith the ta2Jer of the body, and there is 

 evidence that this decrease is due to a dwarfing of the body." 



A short time ago Mr, Boulenger, who guides and encourages 

 me in my work, gave me a copy of this most interesting paper, 

 and suggested that I should find how far these laws apply to other 

 snakes. I have thei-efore made a careful study of our three 

 British snakes, as representing Avidely difiering types subjected to 

 the same climate. 



In all I have found that there is this abridgment of certain 

 longitudinal I'ows of dorsal scales, and that in each of these species 

 the manner in which this takes place is highly charactei'istic. 



On examining any snake with abridged series of scales, one at 

 first concludes that these have been formed by the regression of 

 rows which were complete in the primitive state. All the implicated 

 species, however, are not being subjected to dwarfing iufiuences, 

 as is evidently the case Avitli the Clarter-snakes, and in some 

 groups, notably the Vipers, evolution seems to tend towards 

 enlargement and increased number of scales. To determine the 

 manner in which any given abridgment has been arrived at, it is 

 necessary to consider the scale-formula of the primitive form of the 

 genus, and, in some cases, to make comparisons with individuals 

 having the next highest and next lowest formula. 



1. Tropidonottis natrix has 21 scale-rows upon what may be 

 called the neck, the reduction to 19, brought about by the termi- 



* " Variations and Genetic Relationships of the Garter-snakes." 15nll. 61, Smitli- 

 sonian lust. U.S. National Museum, 1908. 



25* 



