LITTORINA. 363 



writers have described or enumerated it as ranging from 

 Heligoland to the .JSgean, along the sea-board of the 

 Atlantic, Mediterranean (including Algeria), Adriatic, 

 and Black Sea, westward to Madeira and the Canary 

 Isles. 



This is probably the only kind of Littorina common to 

 the north and extreme south of Europe. It congregates 

 in families or clusters, and in dry weather adheres to the 

 rock by means of a membranous film or epiphragm in 

 front of the operculum, of the same nature as that 

 which is secreted by some of the herbivorous Helices 

 and Bulimi. This state of aestivation sometimes lasts 

 many days, during which the little periwinkle appears to 

 fast. The foot is all this time kept withdrawn, in order 

 to prevent any evaporation of the water by which the 

 gill-plume is kept moist and fit for action. The smaller 

 varieties and young of L. rudis are frequently attached 

 in the same manner to rocks beyond the reach of the 

 tide. Bouchard- Chantereaux noticed this singular habit 

 about thirty years ago. Some individuals, which I im- 

 mersed in fresh water for eighteen hours, crawled about 

 vigorously after being restored to the open air. My 

 largest specimens were collected by Mr. Barlee in Arran 

 Isle on the coast of Galway ; they are four lines long. 

 The shell is frequently eroded or fretted, like the lime- 

 stone on which it is commonly found ; for this reason 

 it often appears distorted. The outermost layer of the 

 shell (owing probably to its constant exposure) occa- 

 sionally exhibits in certain parts a ramified or efflores- 

 cent appearance, as if it were permeated by an extra- 

 neous tubular organism. I submitted specimens to the 

 examination of Mr. Berkeley and Dr. Bowerbank. The 

 former thought this appearance might be a condition of 

 some parasitic sponge ; but the latter considered it " a 



