SNAKES OF THE GENUS PITUOPHIS 63 



number from 50 in ruthveni to from 30 to 37 in melanoleucus. Such 

 a tendency toward the fusion of spots is apparent in ruthveni, as 

 shown by the comparative size of anterior and posterior spots, as 

 well as by the partial fusion actually evident in the middle of the 

 dorsum. If on the type specimen of ruthveni this fusion were carried 

 forward and the smaller anterior spots were fused in pairs, the total 

 number of spots on body and tail would be 40, rather than 50, a 

 number only slightly without the known range of variation in the 

 number of spots in typical melanoleucus. In scale characters as well, 

 melanoleucus is easily derivable from ruthveni, the general tendency 

 being a continuation of the dwarfing in length and diameter (ac- 

 companied by a decrease in the number of spots, but an increase in 

 the average ratio of tail length to total length), which is evident 

 from s. sayi to m. ruthveni. 



The possibility of either lodingi or mugitus being phylogenetically 

 intermediate between ruthveni and melanoleucus is precluded in either 

 case by the coloration. In mugitus a decided diminution in color 

 from the condition present in ruthveni is found, in direct opposition 

 to the tendency toward intensification apparent in melanoleucus. In 

 lodingi, on the other hand, the intensification is far greater than 

 that in melanoleucus, since it affects the ground color as well as the 

 spots, and results in a uniformly black dorsum and slate-gray beUy. 

 In addition, lodingi appears to be more specialized than melanoleucus 

 in the proportionately greater tail length and in the dorsal scale 

 formula, in both of which characters it has departed farther from 

 ruthveni than has the latter form. Furthermore, in lodingi the 

 average siun of the ventrals and caudals is higher than in ruthveni, 

 while in melanoleucus it is lower. Thus it seems probable that 

 melanoleucus and mugitus have both been derived directly, but 

 independently, from ruthveni, while lodingi has arisen later in an 

 anomalous position where it is geographically (fig. 33), but neither 

 morphologically nor phylogenetically, intermediate between ruthveni 

 and the two earher derivatives of the latter. 



The probable relationships of melanoleucus and the allied forms 

 may be expressed by the following diagram: 



y melanoleucus 



ruthveni — >lodingi 

 \ 



i^mugitus 



Table 5 lists the specimens of this form that have been examined. 



