SNAKES OF THE GENUS PITUOPHIS 21 



The presence or absence of azygos plates is extremely variable. 

 None of the specimens of the subspecies of melanohucus shows an 

 azygos, but the few specimens available may not accurately represent 

 the true situation in these forms. In other forms one and occasionally 

 two or even three azygos plates are common between the frontal 

 and prefrontals. Occasionally an azygos appears between prefrontal 

 and preocular on either side, and in a few cases in other positions 

 between the head plates. 



Variation in pattern. — The fundamental pattern throughout the 

 genus is a series of large, dark, median, dorsal blotches on a Hght 

 ground. Several additional alternating series of smaller spots are 

 generally present on each side. Marked modifications of this basic 

 pattern appear in the most highly specialized forms of the genus, 

 lineaticollis, on the southern periphery, and the subspecies of melano- 

 leucus, in the eastern part of the range. In lineaticollis, the typical 

 dorsal spots break up into two series of annuU in the midregion, which 

 fuse to form two continuous dorsal stripes on the anterior part of 

 the body. The various patterns typical of the subspecies of melano- 

 hucus may be derived readily from that of ruthveni, the ancestral 

 form of the group, by intensification or diminution of color and 

 fusion of spots, as described hereinafter under the separate forms. 

 In the forms of the melanoleucus group the number of spots, when 

 distinguishable, varies between 22 and 41 on the body and 5 and 10 

 on the tail. In the rest of the genus the number of spots varies 

 between 21 and 94 on the body and 7 and 36 on the tail. The num- 

 ber of spots is sufficiently constant within definite limits in each 

 form to serve as a diagnostic character of considerable importance. 



In correlation with the higher number of caudals in males than in 

 females, the number of tail spots is slightly higher in males in every 

 form. No such correlation can be observed between the higher 

 number of ventrals in females and the number of body spots. 



Except in deppei and to a slight extent in afinis and annectens, no 

 geographic variation in the number of spots is apparent. In these 

 three forms it is in harmony with variation in scale rows, ventrals, 

 and caudals m affinis, but in deppei and annectens the tendency is the 

 reverse of that observable in those characters. The geographic 

 variation noted in these forms is of doubtful significance. 



Affinities. — A close relationship generally has been considered to 

 exist between Pituophis and the genus Arizona. The lack of apical 

 pits and of keels on the scales in Arizona indicates, however, that the 

 affinity between these two genera is not so great as generally beUeved. 



The problem of genetic relationsliips between genera is quite 

 different from that of intrageneric affinities, and in the field of herpe- 

 tology much more thorough researches on the anatomy, embryology, 

 and variations within the genera in question are necessary for the 



