GASTEROPODA. 



Ill 



Family 16. — NA TIC ACE A. 



The animals have an enormously large foot, the front 

 part of it is thick, and drawn over the shell ; the snout- 

 formed head is concealed, the tentacula are remote, the 

 eyes wanting. The shell is oval, globular, or compressed, 

 with the whorls increasing quickly ; the aperture entire, 

 in the shape of a half circle; the outer lip sharp; the 

 operculum small, or a perfect one, or none. — Marine. 



Natic a. Adanson. — Shell smooth, or without an epi- 

 dermis, more or less globular, strong, 

 with but few whorls, and an umbilicus, 

 which is, however, in some species 

 covered by a coating of shelly matter ; 

 outer lip sharp, semicircular, inner lip 

 and inside smooth and thickened ; oper- 

 culum shelly or horny, and sub-spiral. 

 Animal, very large in proportion to the 

 shell, but always perfectly retractile ; 

 head small, furnished with a long trunk, surrounded 

 by a tentacula veil, from which spring two lanceolate ten- 

 tacles, set well apart ; eyes usually absent, when present 

 very minute, and placed beneath the tentacula veil ; foot 

 very large and expanded, rounded at both ends ; men- 

 turn greatly developed, forming a large oblong disk in 

 front of the shell, the anterior portion covering the foot, 

 the posterior rcflexed upon the head and tentacula, so that 

 the tips only of the latter appear above when the animal is 

 walking ; opcrculigerous lobe very ample, reflexed upon, 

 and partially concealing the shell both at the sides and 

 back ; jaws distinct, corneous ; tongue short, linear, each 

 row of teeth upon it consisting of a quadrate broad-based 

 median tooth, with a denticulated apex, flanked by three 



