GASTEROPODA. 



135 



Order II. 

 SCUTIBRANCHIATA. 



The organs of respiration in the animals ot this order are 

 in the form of a comb, and for the most part double ; the 

 tongue is very long, strong, linear, and in a great measure 

 concealed within the intestinal parts, the middle part is 

 many-toothed, and the sides studded with numerous rows of 

 hooks. The shell is either regularly symmetrical, or only 

 slightly rolled up, or cup-shaped. 



Family 1 . — NERITA CEA. 



The animals of this family have the eyes stalked ; no 

 frontal lobes, and no appendages to the sides of the foot. 

 The shell is spiral, globular, conoidal, with no umbilicus ; 

 the columella lip flattened ; the mouth half-moon shaped ; 

 operculum shelly. — Marine and fluviatile. 



Nerita. Lin, — Shell with the spire nearly hidden, or 

 very short; aperture semicircular; 

 inner lip flattened, and frequently 

 toothed ; the outer simple, or 

 waved; no umbilicus; operculum 

 shelly. Animal, with two slender 

 tentacula, with detached eye-bear- 

 ing sustentacula at their external 

 bases; foot rather short, triangu- 

 larly oblong ; structure of the 

 tongue complicated ; each transverse series of teeth has a 

 minute central denticle, flanked by three laterals, of which 

 the first is largest, transversely expanded, and laminar, 



K 4 



