354 LITTORINID^. 



Genus II. LITTORI'NA* Ferussac. PL VIII. f. 3. 



Body stout, twisted into a short cone : head strong : ten- 

 tacles conico-cylindrical and smooth : eyes placed on globular 

 expansions of the tentacles at their outer bases, or sessile : 

 foot oval, rounded at each end, plain-edged : opercular lobe 

 smaller than the operculum, and destitute of appendages. 



Shell rather solid, not umbilicate : spire short : mouth 

 oval, with the lips usually disunited : pillar even, never chan- 

 nelled or grooved : operculum having underneath a process of 

 attachment on or near the nucleus of the spire. 



The presence of these shells in a fossil state affords a 

 useful criterion to the geologist, and invariably indicates 

 littoral conditions. They inhabit only 



" The beachy girdle of the ocean," 



and are seldom found at a greater depth than low-water 

 mark of spring-tides. L. neritoides and some of the 

 varieties of L. rudis take up their abodes above high- 

 water mark, where they probably subsist on Lichina 

 pygmcea and other minute sea-weeds, which cover the 

 rocks in such situations. They have never been ob- 

 served to go down to the sea when the tide comes in. 

 This peculiar habit of truly marine mollusks frequent- 

 ing places beyond the reach of the tide induced Dr. 

 Johnston to make the following quaint remarks on a 

 subject which has of late much engaged the attention 

 of naturalists. After mentioning the case of certain 

 Gasteropods, furnished with gills, that pass so large a 

 portion of their term of life completely out of the 

 water as almost to be amphibious, he says, ' ' The Pfl- 

 tella and Littorina are also good examples. Our com- 

 mon species of the latter genus seem, indeed, to prefer 



* From littus, the sea-shore. 



