No. 433.] SNAKES OF SOUTHERN MICHIGAN. 



13 



between 15.6 and 16.5, inclusive, are reckoned as 16 per cent, 

 those between 16.6 and 17.5 as 17 per cent, and so on. 



It is worth noting that the three sipedons with eyes over 

 19 per cent of the head length and the three crythrogastcrs with 

 eyes over 2 1 per cent are all large males, while the erythrogasti rs 

 with eyes less than 20 per cent are large females. A glance 

 at Table V will show that there is other evidence to indicate 

 that males have slightly larger eyes than females. The differ- 

 ence, however, is hardly sufficient to be easily recognized. 



3. Number of Urosteges. — Reference to Table III will show 

 that the male erythrogaster averages three, and the female four, 

 more urosteges than the corresponding sex of sipedon. This 

 is rather noteworthy in view of the fact that there is no appre- 

 ciable difference in the length of the tail in the two forms. 

 The same point may be illustrated by selecting a few examples 

 of sipedon, giving the total length and the number of urosteges, 

 and placing above each the corresponding specimen of erythro- 



gaster of the 1 

 size. Thus : 



Fry throwster, 

 Sipedon, 



s7pedon, 

 ' Erythrogaster, 

 Sipedon, 

 Erythrogaster, 

 Sipedon, 



vhi. h 



2, 774 



of all . 



the 



" 77 



Although these cases were selected at random, they are purely 

 illustrative, and not at all decisive. Examples might be given 

 showing opposite conditions. Nevertheless, it must be admitted 

 that erythrogaster seems to have, normally, a few more urosteges 

 than sipedon. 



4. Number of Gastrosteges. — In no respect, except color, is 

 there shown such a marked difference between erythrogaster 

 and sipedon as in the number of gastrosteges, a character 

 which is of great importance in distinguishing different species 

 of snakes. It is important to note here that there is no evident 

 between the number of gastrosteges and sex or 



