SNAKES OF THE GENUS PITUOPHIS 7 



tal plate separating the first infralabials at the symphysis of the lower 

 jaw; anterior pair of chin shields longer and wider than the posterior 

 pair, in contact with each other, but separated from the mental by 

 the first pair of infralabials; posterior chin shields usually separated 

 by a few scales. 



The scales are keeled, with the exception of the several lower rows 

 on either side. Apical pits are present in all forms. The dorsal 

 scales are arranged in a series of longitudinal alternating rows around 



Figure 2. — Head of Pituophis melanoleucus melanoleucus: a, From the side; 6, from above. 



the body, which number from 25 to 35 just posterior to the neck, 

 from 27 to 37 in the middle of the body, and from 19 to 25 just anterior 

 to the vent. The lower number of scale rows anterior and posterior 

 to the middle of the body results from the loss of certain definite scale 

 rows. The method of reduction will be discussed below. The ven- 

 tral surface of the body is covered by a single series of transverse 

 scutes, the ventrals, which vary in number from 205 to 262 in the 

 genus. Posterior to the ventral scutes, and just anterior to the vent, 

 is a single anal plate, semilunar in shape. Posterior to the vent are 

 the caudal scutes, which vary in number from 47 to 84 pairs. 



The structure of the hemipenes is so constant throughout the genus 

 that the description of these organs in a typical specimen of any of 



