No. 433-] SNAKES OF SOUTHERN MICHIGAN. 



7 



urostege on one side than on the other, but in such cases the 

 larger number was recorded. A water snake with more than 

 70 urosteges is (in southern Michigan) almost certainly a male, 

 while one with less than that number is almost as surely a 

 female, only three males, out of 49 examined, having less than 

 70. It is interesting to see that the number is not dependent 

 at all upon the size of the snake. The five largest females of 

 sipedon average 62.6 urosteges apiece, and the five smallest 

 average precisely the same, while the five largest males average 

 72.4, and the five smallest, 72.6. 



4. Correlation between Length of Tail and Number of Uro- 

 steges. — It is a noteworthy fact, though quite in accord with 

 what might be expected, that there is a certain amount of cor- 

 relation between the length of the tail and the number of uro- 

 steges. Thus, we find that the males of sipedon which have tails 

 25 per cent of the total length, or longer, average 75 urosteges 

 apiece, while those in which the tail is less than 24 per cent 

 average only 71. The females which have tails 22 per cent of 

 the length or over average 65 urosteges apiece, while those 

 which have tails less than 21 per cent average considerably less 

 than 62. Thus, for each one per cent in the length of the tail of 

 sipedon there are approximately three urosteges, without regard 

 to sex or age. This correlation is not perfect, however, for 

 long-tailed snakes sometimes have a small number of urosteges 

 and short-tailed snakes a large number ; thus, one female with 

 a tail just 21 per cent of the body has 65 urosteges, while 

 another with the tail 22.5 per cent has only 63. Moreover, 

 snakes with tails of the same length sometimes differ greatly 

 in the number of urosteges ; thus in the case of two males 



