VARIATIONS OF GAKTER-SNAKES. 47 



plains and lower mountain slopes are the home of the cactus, mes- 

 quite, yucca; creosote bush, and prickly pear, while the summits and 

 canyons support a luxuriant growth of hardwoods or pines, according 

 to the altitude. 



Specimens have been examined from Tucson and Yuma, Arizona; 

 Duck Creek, New Mexico; Lerdo, Durango and Coyotes, Durango; 

 Guanajuato; Ocatlan, Jalisco; Lake Chalco, Chapultepec, City of 

 Mexico, Chalco, Lake Xochilmilco, and Lerma, Mexico; Patzcuaro, 

 Michoacan; Puebla, Puebla; "South Mountain or Micrado," Vera- 

 cruz; Colonia Juarez, Jimenez, Chihuahua, San Andreas, and 

 Minaco, Chihuahua. From these records the range of megalops may 

 apparently be defined as the entire southern part of the Mexican 

 plateau, extending northward along the western part of the plateau 

 into southern Arizona and New Mexico. Owing to the fact that no 

 real boundary of the tableland exists on the north, no geographical 

 line of demarcation can be drawn here, and we find the species ranging 

 into the open basins of southern Arizona and southwestern New 

 Mexico, as far west as Yuma and to the north possibly to the escarp- 

 ment of the Colorado plateau (the northernmost records being Fort 

 Whipple, Arizona, and Duck Creek, New Mexico). Coues (1875, 615) 

 recorded the species from Fort Whipple, Arizona, and Cope deter- 

 mined his specimens as this form. We are unable either to find 

 these specimens or any others from this locality, so that, in view of 

 the difficulty that has existed in properly distinguishing the garter- 

 snakes of the southwest, this record should be held in abeyance until 

 supported or denied by further evidence. Whether or not the form 

 extends entirely across the northern part of the Mexican plateau, 

 i. e., into the States of Coahuila and San Luis Potosi, remains to be 

 discovered. 



As we stated in the consideration of the environmental conditions, 

 there are no geographic barriers in an east and west direction on the 

 plateau, and megalops probably ranges over the entire area (Fig. 13). 



Variation. — The uniformity of topography is clearly expressed in 

 the homogeneity of scutellation, and the variation when properly 

 determined elucidates many of the questions that have arisen over 

 the division of the form. Bearing in mind that the males have as 

 a rule a longer tail and more ventral and subcaudal plates than the 

 females, and making allowance for the irregularities caused by a 

 marked dominance of either in the averages, it will become evident 

 that a great similarity in scutellation prevails all over the plateau 

 from Puebla to Arizona. The scale rows, only in rare instances, vary 

 from 21-19-17 by the presence or absence of a row; the ventrals vary 

 from 152-173 with an average of about 160-165; the tail length is 

 about .23 of the total length in the males and .21 in the females, and 

 the number of labials oscillates about an averajre of 8 or 8.5. So 



