20 BULLETIN 17 5, UNITED STATES NATIONAL MUSEUM 



are even more irregular and may involve any of the scales except 

 those that lie directly beneath the eye. 



The existence of a sexual variation in the labials and oculars is 

 open to question, although a very slight one seems to occur in most 

 forms. In affinis, vertebralis, and the subspecies of catenifer the 

 average numbers of supralabials and infralabials and of preoculars 

 and postoculars are all higher in females than males, with the excep- 

 tion of the preoculars in c. deserticola, which are slightly higher in 

 males. In sayi, all except the preoculars are slightly higher in males, 

 which is equally true of d. deppei and lineaticollis. In d. jani, all 

 these characters, except preoculars, are higher in females. The small 

 number of specimens studied must be taken into account in the 

 consideration of the last three forms. The same thing, however, 

 cannot be said of sayi, which is represented by 255 specimens. In the 

 three subspecies of melanoleucus represented by specimens of both 

 sexes, supralabials are higher in females and infralabials higher in 

 males, while preoculars are one in both sexes, with the exception of 

 one female melanoleucus and one male mugitus, wliich have 2 on either 

 side. In lodingi and mugitus the postoculars are higher in females, 

 but in melanoleucus in males. Here again the small series of speci- 

 mens must be taken into account. On the whole, the sexual variation 

 in these characters seems to be of little significance. 



The presence of any general tendencies in geographic variation in 

 these scales is equally doubtful. What slight tendencies are evident 

 in forms represented by adequate series of specimens are, however, 

 generally in harmony with the corresponding variational tendencies 

 in scale rows, ventrals, and caudals. 



Variation in other head plates. — The number of prefrontals is one 

 of the most constant scale characters of the genus, and the two pre- 

 frontals of deppei, jani, and lineaticollis distinguish those three forms 

 from the rest of the genus. A single specimen of deppei shows four 

 prefrontals, however, while a very small percentage of specimens of 

 sayi, deserticola, and catenifer have only two prefrontals. 



Another character of extreme constancy within the various forms 

 is the shape of the rostral plate. This, however, shows marked 

 interspecific dift'erences and is therefore of great taxonomic importance. 

 The rostral is invariably at least twice as long as broad in all the four 

 subspecies of melanoleucus; is nearly, but never fuUy, twice as long as 

 broad in sayi sayi; and is slightly longer than broad in sayi affinis. 

 In all other forms of the genus, except occasional specimens of catenijer 

 deserticola, it is invariably at least as broad as long. 



The loreal is usually but not always present and is always longer 

 than high. It is not infrequently divided to form two or even three 

 small scales on one or both sides, in which cases the division is longi- 

 tudinal rather than vertical. 



