THE NAUTILUS. 43 



smallest on these streams was 7.6 mm. alt. by 15 diameter; 



the largest was 20 mm. in diameter. One colony had fine 

 albinos. 



MIGRATION OF ILYANASSA OBSOLETA, LITORINA LITOREA 

 AND LITORINA RTJDIS. 



BY C. H. BATCHELDER. 



The migration of a species is an important event in the his- 

 tory of a brackish water stream. It marks at once, the depar- 

 ture of somebody's food and the passing of somebody else's enemy; 

 it means that the biological relations of the remaining species will 

 be upset and that readjustment must take place. With the 

 passing of an enemy, a species that has lived in seclusion pros- 

 pers, multiplies and assumes other abodes previously inacessible. 

 In new situations new foods become available, and so things 

 change. The snails of a brackish-water stream are as subject 

 to annual migrations as the other inhabitants and some of them 

 are extremely interesting to observe. Migrations of Nassa obso- 

 leta, L. litorea and L. rudis were observed on Oyster river, 

 Durham, N. H. in 1914-15 and the following is an account of 

 the movements. 



The most notable molluscan migrations that I have observed 

 are the annual movements of Ilyanassa obsoleta. During late 

 spring, summer and early fall it inhabits the mud flats exposed 

 at low-tide. This environment becomes impossible, however, 

 during the colder months when ground-ice may form and this 

 forces the snail to migrate to the deeper waters for the winter. 



Migration of Ilyanassa downstream was first observed on the 

 18th of October in 1914 and it had probably proceeded then 

 during five or six days. Migration continued during the next 

 two weeks but the greater number of healthier individuals 

 moved down-stream in the week ending October 25. The rate 

 of their movement appeared to depend somewhat on the tem- 

 perature but no measurements were taken in support of this. 

 The healthy individuals moved out into the deep water of the 

 channel to depths varying from ten to fifteen feet at low tide. 



