98 BULLETIN 17 5, UNITED STATES NATIONAL MUSEUM 



the distance between the mtemasals; frontal undivided, or partially 

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



The dentition is as follows: Mandibular teeth 17 to 21, decreasing 

 slightly in size posteriorly; maxillary teeth 16 or 17, decreasing slightly 

 in size posteriorly; palatines 9 to 11, slightly smaller than mandibular 

 and maxillary teeth; pterygoids 9 to 13, slightly smaller than the pala- 

 tines, and decreasing in size posteriorly. 



The dorsum bears a series of black, brown, or grayish-brown spots, 

 which number 33 to 68 (average 50.7) on the body and 8 to 20 (average 

 13.0) on the tail. When the spots are brownish they are frequently 

 outlined with black or darker brown, at least on the posterior border. 

 Each spot is 2 to 5 scales in length and 9 to 12 scales in width, and the 

 interspaces between the spots are 1 to 4 scales each in length. These 

 interspaces occasionally have a narrow median dark transverse streak, 

 and frequently the light scales, as well as the light scales of the sides, 

 each bear a small median or anterior dark spot. There are three or 

 four lateral series of small dark spots on each side, which alternate with 

 one another and with the dorsal series, and which frequently fuse with 

 one another and with the median series at the edges, forming a kind 

 of syncytium, particularly in the anterior part of the body. The lat- 

 eral spots are frequently partially outlined with black. The belly 

 always bears a series of lateral spots on each side, which are usually 

 separated from one another by from 1 to 4 scutes. In addition to these 

 lateral series, the belly generally is heavily spotted throughout, but 

 occasionally these additional irregularly scattered spots are incon- 

 spicuous or lacking. The ground color of both dorsum and belly is 

 a yellowish white. The top of the head is a reddish brown, more or 

 less dappled with black, and usually each scale of the posterior part of 

 the head and of the neck bears a small black spot. A black streak 

 marks each suture between the supralabials and between the infra- 

 labials. The throat is white (fig. 50). 



Variation. — In many of the characters there appears to be a marked 

 correlation between variation and geographic distribution. On the 

 accompanying graphs the regions that are represented by specimens 

 are numbered along the abscissas, 1, 2, 3, etc., as follows: 



Region L Northern Mexico and Texas. 



2. Oklahoma. 



3. Kansas and southwestern Missouri. 



4. Colorado, Nebraska, southeastern Wj^oming, and the southern part of 



South Dakota. 



5. Northern Wyoming, northern South Dakota, North Dakota, and 



Montana. 



6. Iowa, western Indiana, Illinois, and Minnesota. 



In the number of scale rows (fig. 51), the number of ventrals (fig. 

 52), and the number of infralabials (fig. 54) and postoculars (fig. 55) 

 there is a more or less constant decrease from south to north, which is 



