NERTTA. HALTOTIS. 35 



shells ; others live in fresh water; few inhabit the 

 sea. 



'^ Shell univalve, spiral, brittle ; aperture con- 

 tracted, semilunar, or roundish. The common 

 snail is well known to most persons. Helix ne- 

 moralis, the wood-snail, is very pretty ; some- 

 times it is pink, with brown bands, or plain yellow, 

 or yellowy banded with brown. (Plate 7-) 

 The greater part of this genus consists of shells 

 remarkable for their thin, brittle, and semi- 

 transparent substances. 



" The Nerita genus is not very pretty : (Plate 

 7:) the texture of the shell is in general much 

 thicker than that of HelLv. The shell is spiral, 

 gibbous, pillar-lip transversely truncated, flattish. 

 Seventy-six species. Nerita polUa is a handsome 

 species : those most valued are from the South Sea. 



" We have now lost sight of the pillar-lip, and 

 in the genus Haliotis we find a flat, ear-shaped 

 shell, the spire nearly hidden, the disk perforated 

 lengthways with pores. Species twenty-one. The 

 animals that inhabit the ear-shells fasten themselves 

 so firmly to the surfaces of rocks, that much force is 

 needful to disengage them : during the fine nights 

 of summer, the animal feeds on the herbage that 

 grows on the sea-shore. The sea-ear from New 

 Zealand, and that from California, are superb shells 

 of considerable size. The British species, Ha- 

 liotis tuberculdta, is not uncommon. (Plate 8.) 



