26 



BULLETIN 61, UNITED STATES NATIONAL MUSEUM. 



Fig. 5.— Thamnophis megalops (1098 Field Mu- 

 seum), SHOWING THE SMALL SIZE OF THE FOURTH 

 SUPKALABIAL "WHEN THERE ARE NINE IN THIS 

 SERIES. 



extends from the rostral to some point below the posterior nasal scute, 

 the posterior margin of the second ends under the loreal, the tliird 

 under the preocular, the fourth under the eye, the fifth under the 

 postoculars, the sixth under the first temporal, the seventh under the 

 second temporal, and the eighth under the second scale behind the 



first temporal, which may be 

 termed, for convenience, the lower 

 scale of a third row of temporals. 

 In all of the forms with a normal 

 number of 8 supralabials this 

 arrangement is the rule, and 

 specimens are seldom observed in 

 which the margins of the scutes 

 are outside of the limits of the 

 plates mentioned above, although slight deviations in this way 

 do occasionally occur. Now, when there are 9 supralabials, as 

 in many specimens of megalops, the margin of the extra scute in 

 every case observed is situated entirely within the limits of the pre- 

 ocular (fig. 5). This is always smaller than the adjacent scutes, and 

 is always and only present when there 

 are 9 labials in the series, so that there 

 can be no question that it is the one that 

 is added. 



If now a series of specimens of any 

 form in which the supralabials are nor- 

 mally 7 and occasionally 8 {radix), or 

 usually 7 and frequently 8 (ordinoides) , 



be scrutinized, it will often be noticed that in the specimens 

 with 8 the third is frequently decidedly narrower than the second 

 or fourth (fig. 6). Both in front and behind the eye in such 

 specimens the supralabials are arranged exactly as in those which 

 have normally 8, except that the common suture of the second and 



third occurs near the posterior margin 

 of the loreal, thus narrowing the third 

 plate very decidedly. In these forms 

 when there are but 7 scutes there is 

 either no labial suture below the loreal, 

 the second plate extending from the 

 posterior nasal to the preocular, or the 

 posterior margin of the first labial is 

 moved slightly backward so as to lie under the extreme anterior 

 part of the loreal (fig. 7). There can be no doubt, therefore, 

 that the decrease in the number of supralabials from 8 to 7 is 

 in these cases due to a loss of the third scute and that the 

 method of this loss consists principally of a narrowing of this 



Fig. 6.— Thamnophis ordinoides (1109 

 Field Museum), showing the re- 

 duced THIRD SUPRALABIAL— the FIRST 

 STAGE IN THE REDUCTION FROM 8 TO 7. 



Fig. 7.— Thamnophis sauritus (32972 

 University of Michigan Museum), 

 showing the normal arrangement 



OF THE labial SCUTES WHEN THE 

 FORMULA is 7^9. 



