234 BULLETIN" 114, UNITED STATES NATIONAL MUSEUM. 



The distribution of the scale formulae between the sexes is of 

 interest. It is as follows: 



Formula. 



Male. 



Female. 



Total. 



23-25-23-21-19 



23-21-19 



21-23-21-19 



23-21-19-17 

 21-23-21-19-17 



Totals 



2 



7 

 4 

 5 

 2 



3 

 3 

 3 

 1 

 



5 

 10 

 7 

 6 

 2 



20 



10 



30 





Only 1 female out of 10 ends with 17 rows. A striking feature in 

 which this form resembles multicincta and in which both apparently 

 differ from all other forms in the genus is the possession by many of 

 the same individuals of a maximum of 23 rows and a minimum of 

 17. Three changes from the middle to the posterior end of the body 

 is common when the maximum is 25 rows, but is elsewhere very rare 

 when the maximum is 23, and is quite unknoAvn when the maximum 

 is 21 or 19. In this feature there is certainly support to the propo- 

 sition that pyrrliomelaena and multicincta are more closely related to 

 each other than to any other form of the genus. In other respects, 

 however, the gap between the two seems to be rather wide. Pyrrlio- 

 melaena is in every way a more specialized form. Its scale rows 

 average higher — the maximum in no individual yet examined being 

 lower than 23; there are more ventrals; the tail is the longest in the 

 genus; the lower labials are commonly 10, the upper labials are 

 oftener 8 than in any other form in the genus; the head compares in 

 shape and in distinctness from the neck only with alterna; the denti- 

 tion is the highest for the genus. 



All these features proclaim this form an old and specialized deriva- 

 tive of some more normal representative of the genus. While we do 

 not say that it can not have been derived from multicincta, it is never- 

 theless difficult to conceive of such great structural differentiation 

 where the geographical separation is so little and where the habitat 

 preferences of the two seem to be so similar. It is not impossible 

 that the relationships of multicincta and gentilis may yet prove to be 

 so close as to exclude the former from the possibility of direct rela- 

 tionship with pyrrliomelaena. More material from the Great Basin 

 region is the first essential. We would therefore prefer at present to 

 regard pyrrliomelaena as a relatively ancient and isolated type, 

 because of (1) its high specialization, (2) its great constancy through- 

 out its wide range, and (3) the lack of any form with which it may 

 safely be considered as closely related. 



