12G BULLETIN 17 5, UNITED STATES NATIONAL MUSEUM 



usually present, occasionally divided to form two scales on one or both 

 sides; one or two azygos plates usually present between the frontal 

 and prefrontals, and occasionally a small azygos present on each side 

 between the prefrontal and preocular; rostral always slightly longer 

 than broad, and generally penetrating from J^ to % of the distance 

 between the internasals; frontal usually undivided, but occasionally 

 split for as much as one-half of its length. 



The dentition is as follows: Mandibular] teethj 18| to Jl9, Jdecreas- 

 ing gradually in size posteriorly; maxillary teeth 16 to 17, decreasing 

 slightly in size posteriorly; palatines 8 to 10, slightly smaller than the 

 mandibular and maxillary teeth; pterygoids 10 to 14, sHghtly smaller 

 than the palatines and decreasing in size posteriorly. 



The dorsum bears a series of large median spots, which are a grayish 

 or reddish brown on the anterior part of the body, reddish brown to 

 red posteriorly, and reddish brown to dark brown or black on the tail. 

 Each spot on the body is 3 to 7 scales long and 9 to 1 1 scales wide and 

 is narrowly outlined with black. The spots are quadrangular anteri- 

 orly and saddle-shaped posteriorly or are more or less saddle-shaped 

 throughout; and are in the shape of transverse bars on the tail. On 

 the body they tend to fuse with one another and with the lateral spots 

 at the sides. Three rows of smaller spots are present on each side, 

 which are reddish brown outHned with black and alternate with one 

 another and with the dorsal series. These lateral spots are more or 

 less indefinitely delimited, since they tend to nm together and to fuse 

 with the edges of the dorsal spots. The ground color of both belly 

 and dorsum is a yellowish white. The belly bears on each side a series 

 of small dark spots, which are each 1 to 2 scutes long and are sepa- 

 rated from one another by 1 to 4 scutes. The area between the lateral 

 spots may be almost or entirely immaculate, or may be more or less 

 heavily spotted. The top of the head is a reddish brown, only slightly 

 dappled with dark brown or black, and the throat is white. The 

 sutures between the supralabials and between the infralabials are 

 frequently marked by dark streaks. (Fig. 50.) 



Variation. — In several characters a rather marked geographical 

 variation seems to exist. In the accompanying graphs illustrating 

 the geographic variations of the form the range has been divided into 

 five regions, as follows: 



Region 1. Northern Mexico. 



2. Southern New Mexico and southeastern Arizona. 



3. Central and northern New Mexico, and southwestern Colorado. 



4. Central and northern Arizona. 



5. Southwestern Arizona, southeastern California, and northern Lower 



California. 



In most of the scale characters it may be observed that whatever 

 tendency to variation exists is in general continuous from Mexico to 



