GASTEROPODA. 



107 



Valvata. Midler. — Shell spiral, raised, or flattened ; 

 umbilicated ; whorls round and distinct ; 

 mouth round or nearly so, with the lips ^ 

 united and acute ; a horny operculum, 

 concentrically spiral. Animal, having 

 a distinct head, with a muzzle; and 

 furnished with two very long tentacula, 

 with sessile eyes at the base ; foot two-lobed in front ; 

 branchial plume long, pectinated ; tongue having a central 

 line of broad teeth, which are hooked, and three rows of 

 side teeth. — 11 species ; also fossil. 



Found only in slow running rivulets and ponds, upon 

 plants, in Europe and North America. — There are two 

 British species. 



Family 1 3. — SCALARIA CEA. 



The animals have the head prolonged into a snout, two 

 long slender tentacula, at the bottom of which the eyes are 

 situated ; the tongue has numerous rows of teeth. The 

 shell is for the most part spiral and turret-shaped, the 

 aperture entire, the lips united and thickened. Marine. 



Scalaria. Lam. (Clathrus Oken.) — Shell turreted, 

 sometimes without a pillar ; mouth nearly 

 round, lips united and thickened ; lon- 

 gitudinally banded the whole length of the 

 shell, formed by the thickened lips of 

 former mouths ; operculum horny, spiral, 

 and thin. Animal, having an angulated 

 lunate head, with two approximated long 

 pointed tentacula, eyes immersed at their 

 external basis ; mouth inferior, with a 

 retractile trunk, mantle a rudimentary * "'" 

 siphonal fold, simple edged; foot obtusely triangular, not 



