132 GENUS XXX. NERITA. 



ing- to the shape of half an egg divided lengthways ; the spire 

 of these last is generally lateral and flat. 



Some of the uinbilicated species, as N. Mammilla, have the 

 perforation or umbilicus entirely obliterated when grown to 

 their full size ; the gibbosity, which is commonly found at the 

 umbilicus in this genus, spreading so much as entirely to cover 

 the hole: this seems to be sometimes the case with N. mela- 

 ncstoma, and perhaps with several others. 



The animals of the imperforate species are different from 

 those of the first division, which induced Adanson to make a 

 separate genus of them. Lamarck has adopted Adanson's 

 genera ; and as both the animals and shells are considerably 

 different, this division seems not at all improper. The first 

 division, or the imibilicated sliells, are called by Adanson 

 Natica : the head of the animal is cylindrical, the horns thick, 

 but tapering to a fine point, with the eyes on the external side, 

 at the base or root (^pl. 9. /. 118. a). The imperforate shells 

 are called Nerita by Adanson : the head of these animals is 

 flat and broad ; the horns cylindrical, slender, and pointed ; 

 close to each horn, on the external side, is a short triangular 

 column, with an eye at the tip (/. 119. a). Both animals have 

 opercula. 



Linnaeus describes 25 species, Gmelin has added 47, and 

 Dr. TurtonG. Total, 78. 



Lamarck has divided this genus into two, as follow : 

 Nerita. Shell semi-globose, flattened beneath, and having 



