22 



BULLETIN 61, UNITED STATES NATIONAL MUSEUM. 



Variation in number of labial plates. — The number of labial plates 

 (fig. 2) in the garter-snakes varies from 6 to 9 above the mouth and 8 

 to 11 (very rarely 12) below, each of the numerous forms tending to 

 have a certain definite number, as in the case of the scale rows. For 

 the latter reason the number in each of these series is often used as a 

 diagnostic character, but, owing to the fact that individual variations 

 are not uncommon, this trait has been accorded only secondary impor- 

 tance, while of the two the number of supralabials is considered of more 

 taxonomic importance, as it is noticeably less variable. There seems 

 to have been little attempt, however, to determine the nature of 

 these variations. 



In almost any locality a series of specimens " will reveal differences 

 in the number of these plates. Thus, in proximus, while the labial 

 formula ^ in the great majority of specimens from the same region is 



8/10, there maybe 7 

 upper and '9 or 11 

 lower labials. In 



Rostrdi 





Pr.EDcuUr Seconr^raw 



I I First .Tfiirdrow 



I tem_p_o_r_fl] 



Supris- 



Iflbicjls Tnfrti- 



Fig. 2.— The arrangement of the head plates in the genus Tham- 



NOPHIS, AS represented IN AN INDIVIDUAL POSSESSING THE MAX- 

 IMUM NUMBER OF LABIAL PLATES FOR THE GENUS. 



sauritus the formula 

 is mostly 7/10, but 

 specimens with 8 su- 

 perior and 9 or 11 

 inferior labials occa- 

 sionally occur. 

 Again, ordinoides 

 specimens from the 

 same locality may 

 have 6, 7, or 8 supe- 

 rior and 8, 9, or 10 

 inferior labials. The 

 variability shown by 

 these forms is typical of all of the forms in the genus, and it will be 

 seen at once that the amount of individual variation is very slight. 

 In the case of the lower labials the difference indicated in the speci- 

 mens from the same locality may consist of both a loss and an addi- 

 tion of a scute to the normal number for the locality, but it rarely 

 exceeds this amount. In the case of the supralabials, however, the 

 difference shown by specimens from the same locality usually consists 

 of either an increase or decrease of one scute, it being rarely that the 

 variation occurs both above and below the normal number for the 

 region. 



This apparent definiteness in the variation in the number of upper 

 labials at once suggests the presence of some controlling factor. It 



a The following discussion may be followed on the diagram that forms fig. 3. 

 ^The labial formula is the number of scutes in each series, and is expressed as a 

 fraction; for example, 8/10 means 8 supralabials and 10 infralabials. 



