VARIATIONS OF GARTER-SNAKES. 123 



caiuliils. Tlio tail leiigtii varies from .22-.2r)(), i\\o average of males 

 and females combinetl being about .225. 



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 ])ro])()r- 

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

 cate (1) that in the vicinity of the city of Chihuahua the nund)er of 

 dorsal scale rows, oculars, and ])ossibly the lal)ials reach the maxi- 

 mum for th(^ form, and also for the grouj) and germs; (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 contai't with two in the northern" and southern ])arts of the 

 range. 



Affinities. — In the most southern localities from wliich it is knowTi 

 angustirostris comes in contact with melanogaster. These forms 

 always resemble each other closely in tluit both are characterized by 

 a slender head, small e^e, mostly 8 siiprala])ials 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 

 oi angustirostris there are generally well-defined blotches on the lalnals, 

 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 i)resent, the eye is in contact with two labials, and the dorsal 

 scale formula rarefy exceeds 19-17. 



It may be seen from this that the differences betw^een the two 

 forms are slight except in two ])articulars, the segregation of the eye 

 from the second sid)ocular labial antl in the dorsal scah^ fornuila. In 

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

 wdiich these forms hold in common angustirostris becomes exactly like 

 melanogaster in the fact that the eye is in contact with 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 mnltimaculata (from the San Francisco River) the eyeis in contact 

 with V)ut one labial, l)ut in rtifopunclatum, as already mentioned, it is bounded l)y two. 



