VARIATIONS OF GAETER-SNAKES, 123 



caudals. The tail length varies from .22-. 256, the average of males 

 and females combined being about .22.5. 



It is needless to point out that these figures can not be relied upon 

 to furnish more than a general idea of the scutellation and propor- 

 tions in this form. Still, the more constant characters seem to indi- 

 cate (1) that in the vicinity of the city of Chihuahua the number of 

 dorsal scale rows, oculars, and possibly the labials reach the maxi- 

 mum for the form, and also for the group and genus; (2) that toward 

 the northern and southern limits of the range the number of oculars, 

 especially the preoculars, tends to decrease; (3) that the orbit, which 

 in every specimen from the vicinity of Chihuahua, Chihuahua, is 

 separated from all but one labial by the inferior postoculars, becomes 

 fairly in contact with two in the northern" and southern parts of the 

 range. 



Affinities. — In the most southern locahties from which it is known 

 angustirostris comes in contact with melanogaster. These forms 

 always resemble each other closely in that both are characterized by 

 a slender head, small eye, mostly 8 supralabials and 10 infralabials, 

 nearly the same number of ventral scutes, more than one preocular, 

 the absence of the dorsal stripe, and the presence of a dark median 

 ventral band. The differences between them are that in specimens 

 of angustirostris there are generally well-defined blotches on the labials, 

 the lateral spots are usually in evidence, the lateral stripes generally 

 obsolete, the eye usually in contact with but one labial, and not less 

 than 21-19-17 scale rows. While in melanogaster there are no large 

 blotches on the labials or evidence of lateral spots, the lateral stripe 

 is often present, the eye is in contact with two labials, and the dorsal 

 scale formula rarely exceeds 19-17. 



It may be seen from this that the differences between the two 

 forms are slight except in two particulars, the segregation of the eye 

 from the second subocular labial and in the dorsal scale formula. In 

 regard to the former trait we have already shown that in the region 

 which these forms hold in common angustirostris becomes exactly like 

 melanogaster in the fact that the eye is in contact wath two labials. 

 It will also be shown in the consideration of melanogaster that the 

 number of rows of dorsal scales tends to become as in angustirostris 

 (21-19-17 or 19-21-19-17). The evidence, therefore, favors strongly 

 the view that the two forms intergrade in northern Durango and 

 southern Coahuila, so that it must be adopted for the present, although 

 the relationships of the two forms in the intermediate region should 



a In the type of mullimaculata (from the San Francisco River) the eye is in contact 

 with but one labial, but in rufnpunctatum, as already mentioned, it is bounded by two. 



