32 BULLETIN 61, UNITED STATES NATIONAL MUSEUM. 



limits of this variation overlap considerably owing to the individual 

 variation. Thus, in a large series of radixirom Clay and Palo Alto 

 counties, Iowa, the ventrals in the females range from 148-166, in 

 the males from 160-172. In order to determine whether or not geo- 

 graphic variation occurs in a species it is necessary to note the pro- 

 portion of sexes in the locality averages, since a predominence of 

 either olxscures slight geographic differences. It can be showoi, how- 

 ever, that geographic variation not only exists but that it is corre- 

 lated with the reduction in the scutellation shown above. Concrete 

 examples will be brought out in considering the different groups. It 

 is only necessary to point out here that all of the forms which have 

 a much reduced number of dorsal scales and labial plates have also a 

 proportionately small number of ventral plates, and the opposite. 



On account of their greater variability, therefore, the number of 

 scutes in this series is a trait which is less useful to the student than 

 the number of dorsal scale rows and labial scutes, and, while by no 

 means valueless, is to be used principally to substantiate the evidence 

 of the other characters, when allowance has been made for the dis- 

 turbing factors of individual and sexual variations. 



The variation in the number of subcaudals is of the same nature as 

 that of the ventrals in that the number is greater, as a rule, in males, 

 the extremes of variation in the two sexes overlap, and the individual 

 variation is considerable. In the case of the urosteges, however, 

 the number is not always correlated with the reduction in the scutella- 

 tion, but with the tail length, which may vary to some extent irrespec- 

 tive of the body. The most pronounced case of this is in the inter- 

 grading varieties of the ribbon-snake group. Sauritus in Michigan 

 has a proportionately longer tail and more subcaudals than the south- 

 western form iwoximus, which has more ventrals and labials and 

 attains a much larger size. The character has thus always to be 

 checked up with measurements, but it is not valueless, for when the 

 same proportionate length of tail is maintained the number of urosteges 

 is correlated with the rest of the scutellation. Happily the propor- 

 tion of tail to body length is quite constant throughout the genus; 

 the only exception being the ribbon-snakes, which usually have a tail 

 length of .27-.38 of the total length and are sharply separated by 

 this and the large number of urosteges from the rest of the species 

 in the genus, which have a tail length of .19-.27, the females of the 

 former approaching the males in the latter. 



Variation in number of preoculars and in arrangement of lateral 

 spots. — The number of preoculars has been employed but little 

 in specific diagnoses, although it has been used frequently in 

 descriptions. The explanation for this is to be found in the fact 

 previously stated that in most of the species in the genus there is 

 but a single plate in front of the eye. Certain forms, however, (me- 



