No. 433.] SNAKES OF SOUTHERN MICHIGAN. 



5 



the environment is essentially different from that offered by 

 other creeks near by. 



Differences due to Age and Sex. 



Soon after the collection of statistics was begun, it became 

 apparent that the two sexes could be easily distinguished by 

 external characters, and later on it was seen that the propor- 

 tions of the body were different in very young snakes from what 

 they were in the adults. (Perhaps it ought to be stated that 

 age is assumed to be correlated with size, and snakes less than 

 500 mm. in length are regarded as young.) Before we pass on, 

 therefore, to a comparison of sipcdon and etythrogaster, it is 

 important to make clear the differences which are dependent 

 on age and sex. 



1 . Relative Length of Head and of Tail, and Diameter of Eye, 

 in Old and Young. — If comparison is made between the five 

 largest females and the five largest males, on the one hand, and 

 the five smallest females and the five smallest males, on the 

 other, of sipedon, it becomes clear that in young snakes the 

 head and tail are longer in proportion to the body, and the eye 

 is larger in proportion to the head, than in adults. While this 

 is what might be expected, it is interesting to see how consid- 

 erable the difference is. The form sipedon is used for compari- 

 son because the far greater number of specimens examined 

 makes the contrast more marked. This table shows that if 

 a snake 937 mm. long kept the same proportions when adult 

 that were shown when it was 505 mm. long, it would have a 



