34 BULLETIN 61, UNITED STATES NATIONAL MUSEUM. 



acters are fairly constant in the different forms. For example, in 

 marcianus, ordinoides, and elegans it is usually narrow, while in radix 

 and sirtalis it is decidedly wider, and while it is present in most 

 groups, it is almost universally absent in hammondi, melanogaster , 

 and sumichrasti. It is thus often of considerable value in distinguish- 

 ing the different forms that occupy the same region. Even in its 

 more general aspects, however, it is unreliable as a test of affinities, as 

 is attested by the considerable range of variation in width which it 

 frequentl}^ exhibits in the same form. Indeed, in some instances it 

 tends to be nearly or entirely lost in certain parts of the range of a 

 form in which it is elsewhere distinct, so that an attempt to use it as 

 a test of genetic relationships must result in the grouping together 

 of widely divergent forms. The causes of these variations in the 

 dorsal stripe are difficult to trace without a knowledge of habitats. 

 In certain melanistic forms, such as concinnus, it is present, but 

 narrow. On the other hand, in certain quite pallid types (elegans 

 and eques) it is also quite narrow. As a rule, however, in most 

 melanic forms it is either narrow, obscure, or absent, although it is 

 also obsolete in at least one form in the mountains of Mexico (sumi- 

 chrasti) which is decidedly not melanistic. 



It is hardly possible to plot on a map the regions where the dorsal 

 stripe tends to be lost, for the tendency varies with the form, while 

 in the case of melanism the cause is apparently not always a climatic 

 one. For example, in northeastern United States the genus tends to 

 become decidedly darker colored. In sirtalis specimens this darkening 

 of the general color is associated with the narrowing and frecpient 

 loss of the dorsal stripe, but no such tendency is exhibited by sauritus, 

 which occupies the same region. Similarily, also, although sumi- 

 chrasti on Mount Orizaba has lost its dorsal stripe, it is distinct in 

 scalaris specimens from the same region. The forms of the Mexican 

 plateau all usually show the dorsal stripe distinctly with the excep- 

 tion of melanogaster, in which it is characteristically lacking. In this 

 form, however, it accompanies a melanistic coloration which can 

 not be due to climate, as this is a desert region, but is possibly asso- 

 ciated with the more aquatic habitat of this form. In this connec- 

 tion it is interesting to note that the dorsal stripe in megaloi^s, in 

 which the dorsal is generally very well defintul, is usually very much 

 obscured in specimens taken from Lakes Chalco and Patzcuaro, 

 Mexico, where the form tends to be much darker, and also seems to 

 be quite aquatic in its habits. The conclusion seems justified, there- 

 fore, that the dorsal stripe is probably modified by several factors, 

 one of these being the general darkening of the body color. At any 

 rate, the same peculiarities arise independentl}^ in different forms not 

 directly related, thus invalidating these characters as indications of 

 affinities. 



