152 PROCEEDINGS OF THE NATIONAL MUSEUM vol.91 



This accounts for the hiscutatus group. The record is not so clear 

 for the upsilon group, which has members with more highly modified 

 patterns than the former but (with one exception) without special 

 morphological peculiarities. In this gi-oup two primitive forms are 

 still living — latifascia and fasciolata — of which the former has per- 

 haps the most primitive pattern. Both of these species have very 

 large, few blotches. In distribution they are peripheral to the cen- 

 tral plateau of Mexico. In relation to other members of the group 

 these two stand in much the same position as paucimacidatus and 

 quadruplex do in relation to othei' members of the hiscutatus group. 

 However, it is difficult to reconstruct so plausibly the process by 

 which other members of the upsilon group were derived from 

 latifascia and fasciolata; suffice it to remark that their patterns may 

 have evolved by a splitting and suppression process much like that 

 which occurred in the hiscutatus group. 



Evolution within the upsilon group is made most apparent by 

 changes in the head pattern. The two most primitive types have 

 none, or only a poorly indicated interocular light bar. The least 

 modification in other species is found in tau^ in which the inter- 

 ocular light bar is generally complete, and an indentation of the dark 

 head color along the parietal suture is evident. T. collaris repro- 

 duces this head pattern, and with tau delimits an extensive geo- 

 graphic range completely peripheral to the central plateau. Since 

 increase in number of blotches seems to be the trend in the upsilon 

 group, collaris with few, broad blotches is conceived to be more 

 primitive than tau. It is noteworthy that the opposite extreme 

 (from collaris) in number of blotches in tau occurs in Michoacan, 

 which IS also the farthest extreme from collaris geographically. 



The central-plateau species, upsilon^ was obviously derived from 

 tau or its near ancestor, as its head pattern, with a Y-shaped parietal 

 mark, is clearly derived from that of tau. In number of blotches 

 it remains very similar to the latter. 



The end form in the upsilon group is mlkinsonii, in which are 

 apparent the extremes in reduction of head pattern and of body 

 blotches. The latter is not evidenced by trends in other species of 

 the group, although it is generally the case that multiplication in 

 number of blotches is followed by a decrease in their size. The sim- 

 ple 3-spot head pattern of juvenile vilkinsonii, however, is the end 

 result of the general trend, observed in other species, toward enlarge- 

 ment of the light areas of the head and consequent reduction in size 

 of the dark areas. 



The body pattern of vilkinsonii is highly suggestive of the pat- 

 tern of Lampropeltis leonis, which is fairly certainly known to have 

 been derived by suppression of alternate blotches. This similarity at 



