174 UNIVALVES NERITA. 



the most common variety of the latter shell is entirely 

 white, and has the appearance of porcelain ; but the rarer 

 inclines to a brownish orang-e. 



The species which constitute the next division are 

 imperforated and toothless: of these the N. corona, 

 which has its whorls crowned with spines of unequal 

 length, and the N. fluviatilis, may be adduced as ex- 

 amples. 



The species of the third division are distinguished 

 from those of the preceding, by having one or both of 

 the lips toothed. The N. pulligera, N. polita, N. un- 

 dulata, N. exuvia, and N. chamaeleon, are characteristic 

 of the several families. 



The most beautiful species of the genus is the N. 

 polita, of which the most rare variety has three or four 

 bright crimson bands, on a dark mottled ground, run- 

 ning- in a parallel direction with the convolutions of the 

 shell; they are frequently worn as ornaments by the 

 Indians. 



The different species of Nerita are principally pro- 

 duced in the African, American, Indian, and European 

 seas. 



This genus has received the name of Nij/)/t)jc 

 from its species having been supposed by the antient 

 naturalists to have the power of swimming in the 

 ocean. 



