Nerodia cyclopion and N. floridana 
85 
The first phase of the analysis tested the calibration group 
with stepwise discriminant analysis. The best discriminating vari- 
ables varied by sex, so in all analyses the sexes were treated sepa- 
rately. All head scale measurements were entered into the analysis 
program; those characters which did not contribute significantly to 
the discrimination of the two taxa were discarded. The program, 
using these data, then assigned each specimen to a species group 
{cyclopion or floridana) and calculated the probability of error in 
this assignment. The second phase then tested the specimens in the 
holdout group using the parameters established with the calibration 
group data. 
Fig. 1. Views of the head of Nerodia cyclopion illustrating the method 
of measurement of head scales. The characters are (1) loreal dorsal length 
(LD), (2) loreal ventral length (LV), (3) muzzle width (MW), (4) internasal 
length (IL), (5) prefrontal length (PF), (6) muzzle length (ML), (7) frontal 
length (FL), (8) frontal extension length (EL), (9) parietal length (PL), 
(10) anterior genial length (AG), and (11) posterior genial length (PG). 
