MOLLUSCA. 261 



recently engaged in that compilation, I feel that it would 

 be out of place to enter into a very elaborate history of 

 this class in the present book ; but a few interesting ex- 

 amples may be given. « 



LiTTORIXA LITTOREA. 



On entering the Society's Fish-house, among the first 

 objects that attract notice are a number of snow-white, 

 rounded, or oval discs, studding here and there the sides 

 of marine tanks. Going a little nearer you perceive behind 

 each of them the head and shell of a common Periwinkle. 

 Crawling up tlie glass sides till they arrive near the water's 

 edge, they will often remain without motion for a consi- 

 derable length of time, holding on by their white sucker- 

 discs. And sometimes they rise above the water, prefer- 

 ring to remain dry for a time, in imitation of their natural 

 habits. When observed in the latter position, they are 

 often ruthlessly pushed down by the keeper. On my ask- 

 ing whether the poor Molluscs, accustomed as they were 

 to be left dry on the rocks by the receding tide, could 

 really live always immersed as they seemed condemned to 

 do here, the keeper replied by pointing out to me hun- 

 dreds of tiny Periwinkle fry, with transparent shells, to 



