VARIATIONS OF GARTER-SNAKES. 



49 



one specimen has been (^Xiiinined with more than 21-10-17 rows, 

 while about the City of M(>xic() and in Puebla a considerable j)ropor- 

 tion have the reduced formula of 19-21-19-17, and in three specimens 

 from Veracruz one has 21-19-17, one 19-21-19-17, and the third 

 19-17, the smallest number for the species. It is also of interest to 

 note here that the type of Jiavilahris wdiicli was from Veracruz 

 (State) was said to have had 19 scale rows (Cope, 1866, 306). In 

 the diagram that constitutes fig. 14 we have attempted to represent 

 gra])hically this geographical variation in the num])er of scale rows. 

 The number of specimens at our (Hs])osal may perhaps seem too 

 small u])on which to base conclusions as to the variations in the 

 number of scale rows, since it has often been affirmed that the number 

 may vary a row or two in the same locality. Bearing in mind, how- 

 ever, the narrow limits of individual variation shown in our locality 



Qi-2J-2hl9-l7 



21-19-17 



19-21-19-17 



19-17 



records and indicated on our (hagram, it may be clearly seen, even 

 from this limited .series, that, although slight, the decrea.se in the 

 number of rows from Patzcuaro southward is entirely definite; while 

 a similar reduction is indicated to the northward that may or may 

 not be confirmed by larger series. 



In the number of labials, as in the case of the dorsal rows, the 

 question of sex may be ignored and the averages plotted directly, as 

 has been done on the diagram, fig. 15. By com])aring the tables it 

 wall be seen that although the arithmetical mean is quite constant, 

 it is not uncommon to find 9 supralabials and 10 infralabials in 

 Chihuahua; that no specimen north of Patzcuaro has been observed 

 with less than 8, and that south of tliis locality 9 labials very seldom 



