VARIATIONS OF GARTER-SNAKES. 31 



4/5, 3/4, 8/10; (2) that the loss of the scutes in both series takes place 

 first in front and then behind the eye, which retains the same position 

 in regard to the superior and inferior plates; (3) that as the reduction 

 consists of a decrease in the size, and final loss, of the disappearing 

 scutes, without a corresponding increase in the size of the remaining 

 scutes, a contraction of the mouth is necessarily associated with the 

 decrease in the number of labials. We can even go farther than this, 

 however, for in these snakes the posterior angle of the mouth extends 

 practically to the posterior angle of the jaw, which in turn marks the 

 extreme posterior hmit of the head. This is shown externally by the 

 fact that the cephalic plates never extend beyond the posterior angle 

 of the mouth. Since the number of labials is correlated with the 

 length of the mouth, which extends the entire length of the head, it 

 logically follows that the number of labials is also an expression of the 

 head length. 



It is very difficult to prove the existence of any geographic or racial 

 differences in the length of the mouth or head by actual measure- 

 ments, as these differences must in most instances be very slight, as 

 indicated by the limited range of the variations in the number of 

 labials. It is apparent, however, that such differences exist, for two 

 forms, butleri and ordinoides , are characterized by the noticeably small 

 size of these features, and it is to be noted that besides being the only 

 forms in the genus with a decidedly small mouth and head, these 

 forms also possess the smallest number of labials, and if a comparison 

 be made between these rather extreme forms and their nearest rela- 

 tives, radix and elegans, a difference in the length of the head and 

 mouth may be readily seen. 



From the above discussion of the method of variation in the number 

 of labials the law of labial reduction for the genus may be formulated 

 as follows : The differences in the number of labial j^lates in the garter- 

 snaJces is the result of the loss of corresponding superior and inferior 

 labial scutes, the loss taMng place both in front and behind the eye, thus 

 resulting in a shortening of the Tnouth and head toward this organ. 



Variation in number of ventral and subcaudal plates and in propor- 

 tionate tail-length. — As • previously stated, there is a single series of 

 large scutes on the belly and two rows of smaller ones on the ventral 

 surface of the tail. The number of scutes in each of these series has 

 been considered of systematic importance, and it is true that each form 

 has a definite number within limits. The range of individual and 

 sexual variation is so great, however, and the racial differences gen- 

 erally so slight that large series must be examined from each locality 

 to determine geographic variation. 



The number of ventrals is apparently less variable than the sub- 

 caudals. There is some sexual difference, as may be determined by 

 an examination of a series of specimens from any locality, but the 



