16 BULLETIN 17 5, UNITED STATES NATIONAL MUSEUM 



that in each case the reduction is due to the loss of certain definite 

 scale rows, and although different rows are involved in the various 

 genera, the method is typical and constant in each genus. 



The examination of a large number of specimens of Pituophis 

 reveals that in this genus also a typical system of reduction is normally, 

 although not invariably, followed. Thus, in the reduction from 33 to 

 31 scale rows the eighth row is normally lost almost simultaneously on 

 each side; in the reduction from 31 to 29, the seventh; and so on. The 

 same rows are lost anteriorly as posteriorly in each case, in comparable 

 reductions. A composite of the normal reductions that occur in the 

 genus taken as a whole may be expressed in the following numerical 

 diagram, in which the upper series of numbers represents the numbers 

 of scale rows, and the lower series the lateral rows lost on either side 

 in the corresponding reductions: 



37 35 33 31 29 27 25 23 21 19 

 988776655 



The lower series of numbers represents the lateral rows involved in 

 reduction when counted from the ventral series each time, as is usually 

 done. If a hypothetical individual representing the entire range of 

 variation from 37 to 19 rows is considered, it will be seen that pro- 

 ceeding posteriorly from the region in which 37 rows are found, the 

 first row lost will be the ninth, the second the eighth, the third the 

 tenth, etc., the loss alternating from one side of the ninth row to the 

 other. This is shown in figure 10, in which the Roman numerals repre- 

 sent the actual number of each row, and the small Arabic numerals 

 on the scales represent the order in which the rows are lost. Expressed 

 numerically the actual number of each row lost is as follows: 



37 35 33 31 29 27 25 23 21 19 

 9 8 10 7 11 6 12 5 13 



As mentioned above, individual variations from this typical se- 

 quence occur. The commonest variation is the loss of the vertebral 

 row, or of a single lateral row, instead of two simultaneously, and in 

 either case results in an even number of rows around the body for some 

 distance. In a genus where the range of variation is as great as in 

 Pituophis, a greater degree of individual variation is to be expected 

 than in a genus such as Thamnophis or Coluber, where the range of 

 variation is much more limited and a much smaller number of scale 

 rows is involved. 



From a comparison of the numerical diagrams above with that illus- 

 trating the method of reduction in Thamnophis, it is evident that the 

 series of rows involved in reduction in Pituophis overlaps and completes 

 the corresponding series for Thamnophis, and that the same rows are 

 lost in each comparable reduction in the two genera. Blanchard 

 (1921a, p. 10) has called attention to the similarity in the methods of 



