24 



LITTORINIDiE: 



This group consists of Molluscs, living in the sea or 

 in brackish water, having close affinities with the members 

 of the last family, but cliiFering in several particulars, 

 among which the form of the operculum is conspicuous, 

 since instead of being multispiral, or formed of numerous 

 concentric layers, it is composed of a spire of few and 

 rapidly increasing turns. All the shells of this family 

 have entire mouths, but they differ greatly in form in 

 the different genera, varying from discoid to lengthened 

 cones. The animals are bisexual ; they have muzzle- 

 shaped heads provided with tentacula and sessile eyes. 

 Their tongues are long and armed with transverse bands 

 of teeth, each row consisting of a broad and hooked 

 central denticle flanked on each side by three oblong, 

 hooked laterals or uncini. The branchial plume is single. 

 The foot has a distinct linear duplication in front, and 

 a groove along the sole. The form and apj^endages of 

 the operculigerous lobe afford important generic distinc- 

 tions. There are no neck-lobes or lateral cirrhi. The 

 mantle exhibits traces of a rudimentary canal, or respi- 

 ratory fold. 



Members of this group inhabit all regions of the sea, but 

 by far the greater number live near shore, and a very con- 

 siderable portion of them are found only between tide- 

 marks. 



