68 BULLETIN 61;, UNITED STATES NATIONAL MUSEUM. 



mony witli tlie absence of geographic barriers, is expressed in scutel- 

 lation, tail length, and color. There are, however, variations both 

 in scutellation and color. The color varieties are not individual or 

 sexual, but seem to be partly at least associated with the dampness 

 of the environments. The variations in scutellation and tail length 

 are in the nature of a scarcel}^ perceptible decrease toward the south. 

 This reduction, as shown by our diagrams, is a general one, and 

 involves the number of dorsal scale rows, ventral and subcaudal 

 plates, and supralabials, being best marked in the case of the dorsal 

 scale rows. 



AJfinities. — If we can base our faith upon the position or the lateral 

 stripe there can be no doubt that megalops is a member of the Radix 

 group, since there is never any doubt of its position being upon the 

 third and fourth row^s, for the greater part of the length. Although 

 posteriorly this stripe seems to descend to the second and tliird rows, 

 there has not been a single specimen examined or recorded in which 

 it departed from the third and fourth rows anteriorly, even in those 

 specimens with a reduced scutellation. It is in tliis respect sharply 

 defined from all other forms on the plateau of Mexico, only to the 

 nortliward (southern Arizona and New Mexico) coming in contact 

 with a similar form. 



MARCIANUS.a 



Description. — We have seen that the arid basins of southern Ari- 

 zona and New Mexico are inhabited by one species of the Radix 

 group, the Mexican megalops. It is also the home of another char- 

 acteristic and well marked form, known as marcianus, which is prob- 

 ably entitled to the distinction of being the most palhd form in the 

 genus. This species was described by Baird and Girard in 1853 (1853, 

 36-37) from a specimen taken near Cache C^reek in what is now Okla- 

 homa, and is characterized by the presence of 21-19-17 dorsal rows 

 of scales, 8 supralabials, about 155 ventral plates, about 68 subcau- 

 dals, and a tail length of about .23 in the males and .22 in the 

 females. The lateral stripe is very light, on the third row of scales 

 only anteriorly, posteriorly being upon the second and third rows. 

 The general ground color is light brownish yellow, which is in marked 

 contrast to the l)lack markings. In view of the fact that it has for 

 years been considered a subspecies of elegans, it may appear strange 

 to some herpetologists that this species should be considered a mem- 

 ber of the Radix group. We hope, however, to be able to justify 

 our position by showing that there is in realit}^ less difficulty in refer- 

 ring it here than to the other groups in the genus. 



« Thamnophis marciamis (Baird and Girard), Catalogue of North American Rep- 

 tiles, 1853, pp. 3G-37. Includes the Eulse.nia dcgans iiiarciann of later writers, and 

 E. nigrolateris Brown (1889, pp. 421-422). 



