TEIID LIZAr.DS OF THE GENUS dSrEMIDOPHORUS 257 



population Avhich gave rise to the northern groups, and likewise for 

 the later isolation of portions of this assemblage to allow the further 

 genetic advances tliat are to be seen in Cnemidophorus at the present 

 time. 



It is difficult to surmise the exact geological time at which the 

 differentiation of the sexUneatus group from the lemniscatvs and 

 (U'pp'il groups occurred, but it ])robably did not take place before the 

 early Pliocene. Aside from what has already been noted, a more 

 recent evolution (than that of the deppii group) for the sexUneatus 

 group is suggested by its more northern range. In spite of the 

 fact that the hardy protostock of the sexUneatus group, as exempli- 

 fied by the modern gulans^ is the only satisfactory hypothetical an- 

 cestor of the iessellatus and hyperythrus groups, the main evolution 

 within the sexUneatus group itself has been apparently very recent 

 (Pleistocene). In fact three of the four members of the sexUneatus 

 group are noAv intergrading extensively in the Southwest. These 

 are the central gularis^ the eastern sexUneatus^ and perplexus of the 

 Sonoran region. The latter has given rise to a unique variant^ 

 lahkdis^ which now inhabits northern Lower California. This liz- 

 ard is of considerable importance in explaining the genesis of the 

 hyperythvus group as will be revealed later. 



The tesscllatus group is obviously descended from the sexUneatus 

 group, from which its scutellation and color pattern may be easily 

 and logically derived. As shown in the summary of the tesseUatus 

 group, maxinni'S, and tesseUatus retain more primitive characters 

 than any other forms of the section and are, therefore, the only pos- 

 sible prototypes. 



In the possession of four supraoculars and in the occasional reten- 

 tion of bluish as a suffusion on the ventral surface of the young, 

 these forms show an affinity Avith r/nlaris, the plastic prototype of 

 the sexUneatus group mentioned above, but in the constant presence 

 of small or slightly enlarged granules on the back of the forearm and 

 in the possession of lateral tessellations, cross-bars or irregular 

 stripes in the young they present a decided variation from this 

 spotted racerunner. Although the approach to yularls is usually 

 closer in typical maxhnus than in typical tesseUatus^ there is little 

 doubt that both of these forms had a common origin because of their 

 marked resemblances to each other. The transient ancestral popu- 

 lation of the two forms may have come from the north at about the 

 same time, in which case both are now the remnant of a common 

 stock, of which tesseUatus^ lying nearer to the point of origin, has 

 proved to be the most progressive element. 



Tliere are nine forms in the tesseUatus group, ?naa}hnus, tesseUatus, 

 and seven others. Only one of the latter, ruhidus, occurs on the main- 

 land. Its slight colorational differentiation from tesseUatus and its 



