48 



BULLETIN 01, UNITED STATES NATIONAL MUSEUM. 



closely is this unity preserved over the entire extent of the range that 

 it is only by carefully plotting the data from large series that any 

 definite trend in the variations can be observed. When this is done, 

 however, there is seen to be a distinct although slight decrease in 

 scutellation and tail lenuth toward the south. 





^■t^^ 





J- \ . -ii. * ♦ ^M" M 



Fig. i:?.— Distrihution of Tiiamnophis megalop.s, as indicated by the locality records. 



From Arizona to Patzcuaro, Michoacan, no specimen has been 

 examined with less than 21-19-17 scale rows, but 6 out of 45 speci- 

 mens from the intervening region exhibit a tendency to have more 

 than this number, in the presence of an extra row on either side for a 

 varying distance, thus making the formula 21-23-21-19-17, the 

 extra row never extendim;- to the head. South of tliis localitv but 



