VARIATIONS OF GAETEE-SNAKES. 25 



also in the size of the mouth. While this is true it does not follow, 

 however, that a difference in the size of the mouth is of necessity 

 associated with a difference in the numher of labials, for it is con- 

 ceivable that the same result might be attained by an increase 

 or decrease in the size of the individual scutes, without disturbing 

 the number, or the number of plates in each series might be 

 increased or decreasetl by division or fusion without modifying 

 the size of the mouth. It is necessary, therefore, to determine the 

 method by which variation in the number of labial plates is brought 

 about before the significance of the differences can be discovered. 



I may be taken to task for asserting that variations in the number 

 of labials are caused by a decrease in the size and ultimate disappear- 

 ance of certain plates, for instances of the division and fusion of 

 individual scutes will have been recognized by anyone who has han- 

 dled a considerable number of these snakes. If, however, the fre- 

 quent abnormalities, to which all large scales in snakes are subject, 

 be ignored, we believe that it can easily be demonstrated that the 

 normal method of variation 

 in the labials is associated 

 with the decrease in size and 

 ultimate disappearance of 

 certain plates. 



As was stated in the fen- ^^^- 4— thamxophis megalops (io98 field museum) 



, , . . „ , ^ , SHOWING THE NORMAL ARRANGEMENT OF THE LABIAL 



eral description of the scutel- scutes when the formula is 8 lo. 

 lation in the genus, the eye 



rests directly upon the supralabials, the orbit being above the suture " 

 of two adjacent scutes, fig. 2. These labials are said to "enter the 

 orbit," and the particular scutes thus situated are often mentioned 

 in diagnoses. Thus, in proximus the fourth and fifth labials enter 

 the orbit, in sirtalis the third and fourth, etc. If, however, the rela- 

 tive position of the eye to the supralabials be investigated in the differ- 

 ent forms, in connection with the number of scutes in this series, it 

 very soon develops that the general statement can be made that in all 

 of the forms of garter-snakes with more than 7 sujyralahials the orhit is 

 hounded helow hy the fourth and fifth scutes of this series; in those 

 with 7 or less it is the third and fourth that enter the orhit. 



This may mean one of two things, either the position of the eye 

 is constant relative to the two scutes which bound its orbit, and the 

 difference in the number of labials occurs anteriorl}^, or the varia- 

 tion in the number of labials has taken place behind the eye and 

 has been associated with a change in the position of the orbit. 

 If the position of each superior labial be examined on an}' snake in 

 which there are 8 in the series, it will be found (fig. 4) that the first 



oin angustirostris, in which the orbit is separated from all but one labial by the 

 forward prolongation of the lower margin of the lower postocular,^the relative posi- 

 tion of the eye is the same, as it is still directly above the suture of two labials. 



