MOLLUSC A OR UNIVALVES. 195 



are very common in the West Indies, Moluc- 

 cas, &c. Some naturalists unite this and the 

 preceding genus, as their form is very similar ; 

 but Mr. Gray describes their opercula as quite 

 different. That of the Neritina is solid, shelly, 

 and furnished with a thin, flexible, outer edge ; 

 the Neritce have a horny operculum, covered 

 on both sides with a hard shelly coat, which 

 may be separated from the horny part. 



62. Natica. — Shell smooth, or without an 

 epidermis, having an umbili- 

 cus, which, however, is in 

 some species covered by a 

 coating of shelly matter; 

 outer lip sharp, inner lip 

 and inside smooth ; opercu- 

 lum shelly. Animal, head 

 very large, having two tentacula with eyes at 

 the base ; foot large and thin. — There are 45 

 species recent, all marine, and 1 1 fossil. 



Found on the coasts of England, and also 

 those of India, China, New Holland, &c. 



o 2 



