VARIATIONS OF GARTER-SNAKES. 19 



niaxiiniim number of rows on the different parts of the body." 

 Simihirly also in variable forms that have a maximum number 

 of 21 rows, the fifth is often dropped anteriorly as well as pos- 

 teriorly, giving a formula of 19-21-19-17. In the forms having a 

 maximum of 19 rows the formula may be 17-19-17-15, with the 

 fourth row shortened anteriorly as well as posteriorly, while in those 

 in which 17 is the maximum it is also the fourth that is dropped 

 anteriorly when there are but 15 anteriorly. The abridged row 

 when sliortened anteriorly varies in length, and may extend nearly 

 to the head or be abbreviated so as to include but a few scales on 

 the middle of the body. This suggests at once that among the 

 specimens in a given locality tlio^e in which a certain row is dropped 

 anteriorly as well as posteriorly represent an intermediate condition 

 between those in which this row is continued to the head and those 

 in which the maximum number is one row on either side less than 

 the maximum for the locality, the decrease being brought about by 

 the loss of the first abriflged row in the maximum formula. To 

 illustrate, individuals of radix from the vicinity of St. Louis, Mis- 

 souri, may have the formula 21-19-17, 19-21-19-17, or 19-17, the 

 length of the fifth row varying in the specimens with 19-21-19-17 

 rows from a few millimeters to include the entire anterior half of the 

 body, and we can only conclude that the formula 21-19-17 is a 

 result of the continuation of this row to the head, while in specimens 

 with 19-17 rows it is entirely lacking. 



It is thus the first abridged row that is shortened and lost in the 

 decrease in the number of scale rows, and as this row, as we have 

 ])reviously seen, generally ends a little beyond the middle of the 

 body, it is evident that most of the shortening must take place at 

 the anterior end of the row. There is, however, a posterior shortening 

 of the second abridged row, for upon the disappearance of the first 

 short row the secontl abridged series becomes the first, and its pos- 

 terior end must move up toward the middle of the body to retain the 

 symmetr3^ This variation in the maximum number of scale rows 

 may be summarized as follows: The decrease in the number of rows of 

 dorsal scales among the garter-snakes of a given locality is due to the 

 anterior shortening and subsequent loss of the first row of scales that is 

 dropped posteriorly in each snake having the next higher scale form ula. 



I have been unable to discover any sexual variation in the 

 number of scale rows, but in many cases an examination of large 

 series reveal distinct geographic differences in this regard. Thus in 

 the great plains region radix exhibits the scale formulas 21-19-17 

 or 19-21-19-17, but in the prairie region to the east a decidedly 

 larger proportion of specimens have 19-21-19-17 rows, while near 

 the eastern limits of its range the formula 19-17 is occasionally 



" In the following paper the rows will be numbered from the gastrosteges each time. 



