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THE AMERICAN NATURALIST. [Vol. XXXVII. 



tail 6 mm. longer, a head 4.6 mm. longer, and an eye nearly 

 half a millimeter larger than it does have. While the difference 

 in length of tail is thus rather small, the difference in head and 

 eye is very considerable. 



2. Relative Length of Tail in Males and Females. — If com- 

 parison be made between the males and females of either sipe- 

 don or erythrogaster, the difference in the proportion of tail and 

 body in the two sexes is very marked. 



Table II. 



The above table includes all of the water snakes examined in 

 which the tail was uninjured. It will be seen that while the 

 males average very much smaller in size, the tail is very much 

 longer than in the females. Indeed, in both sipedon and 

 erythrogaster the maximum tail measurement for a female falls 

 short of the minimum for a male, while the average for a 

 female is approximately 3 per cent less than for the male. It 

 thus appears that a snake 800 mm. long will be found to be 

 a male if the tail is over 180 mm., and a female if the tail is 

 less than 180 mm. Ordinarily a male snake 800 mm. long will 

 have a tail not less than 24 mm. longer than a female of the 



3. Number of Urosteges in Males and Females.— If a compar- 

 ison be made between the number of urosteges in males and 

 females, it becomes evident that here again there is a marked 

 sexual difference. 



These numbers refer to the urosteges of one side only, the 

 actual number being double the above, since these plates are 

 arranged in alternating pairs. Occasionally there is one more 



