28 



BULLETIN 61, UNITED STATES NATIONAL MUSEUM. 



Fig. 11.— Thamnophis radix (1.55 

 RuTHVEN Collection), showing 



SIXTH SUPRALABIAL NABROWER 

 THAN USUAL AND ENDING ON THE 

 FIRST INSTEAD OF THE SECOND 

 TEMPOR.\L. 



the fusion of the penidtimate and antepenultimate scutes. Wliile 

 such a fusion undoubtedly exists, however, it constitutes but a part 

 of the story; for, if simply a fusion of these scutes took place, the 

 fifth, in individuals with G labials, should extend from the p^ostocular 

 to the second temporal, wliile, as it is, it only extends a little beyond 

 the middle of the first temporal, being thus but slightly larger than 

 in specimens with 7. A reduction in the size of the penultimate scute 

 must therefore take place at some time during the process of reduc- 

 tion in the number from 7 to 6. A series of specimens of hutleri and 



radix from the common boundary of their 

 range, where, as we have seen, a reduction 

 in the number of labial plates from 7 to 

 6 takes place, will illustrate tliis process in 

 all of its stages. Thus, in most of the 

 specimens with 7, the penultimate scute 

 extends from the anterior part of the first 

 temporal to the second row of temporals, 

 the normal arrangement for this number 

 of labials (fig. 9). In other specimens (fig. 11) this labial extends 

 only from the anterior part of the first temporal to beyond the 

 middle of tliis scute, w^hile in still others the reduced penultimate 

 scute is partially fused with the antepenultimate one (fig. 12). As 

 the posterior margin of the penultimate plate, in specimens with 

 6 labials, generally comes in contact with the first temporal at about 

 the place where the posterior margin of the 

 sixth occurs in the specimens in which it is 

 much reduced, the next step in the reduction 

 of tliis plate would seem to be its entire fusion 

 with the preceding scute to leave 6 (fig. 10)." 

 Therefore the reduction in the number of labial 

 scutes from 7 to 6 in the garter-snalces is brought 

 about by the narrowing and loss of the sixth; and 

 as there is not a corres])onding increase in the 



size of the neighboring scutes, this decrease in the number of labials 

 must be accomjMnied by a shortening of the mouth behind the eye. 



From its apparent general occurrence among the diff"erent forms, 

 the above described method of variation in the number of supra- 

 labials may be formulated in the following general terms: A reduc- 

 tion in the number of swpralabials takes place in the garter-snalces as a 



ol have previously noted (Ruthven, 1904, 294) that when there are 7 supralabials 

 in hutleri, the extra one is apparently formed by the division of the fifth. The expla- 

 nation for this is now shown to lie in the method of reduction in the number of these 

 scutes from 7 to 6. 



Fig. 12.— Thamnophis hut- 

 leri, SHOWING the last 

 stage IN THE loss OF THE 

 SIXTH SUPRALABIAL— THE 

 FUSION OF THE REDUCED 

 SCUTE WITH THE FIFTH. 



