4 GASTEROPODA. 



forming a hinge on the inner lip, making this genus the 

 connecting link between the univalve aud bivalve shells. 

 It contains only one genus — Neritina. 



Tribe II. — PALUDiNiDiE. 



Animal — With a lengthened broad ovate mouth; head bear- 

 ing two tentaculeiB, which are long, slender, and setaceous 

 at their tips ; at the outside of their base, the pedicels 

 on which the eyes are placed subtend; gills single, enclosed; 

 tongue very short with a series of transverse denticles ; 

 foot triangidarly oblong, bearing on a rounded lobe the 

 operculum . Bisexual. Male organ in the right tentacle, 

 causing these tentacles to be of unequal length in the 

 males. 



Shell — Conical or turbinated, with a produced spire, covered 

 with a coloured epidermis ; mouth ovate or nearly round; 

 peristome thin, entire; operculum corneous, with a nucleus 

 at the centre expanding in concentric rings. This tribe 

 contains three genera — Paludina, Bithinia, and Valvata. 



Tribe III.— Littorinid^. 



Animal — With elongated trunk ; head muzzle-shaped, bearing 

 two rather short subulate tentaculce ; the eyes sessile 

 on the outerside of their bases ; no lobes on neck ; the 

 lobe of the operculum, without filamentary process; tongue 

 with dissimilar lateral elements armedwith incurved teeth; 

 foot rounded at both ends. 



Shell — Solid, turbinate, spire produced ; mouth ovate not con- 

 tinued; lip of columella appressed; outer lip with sharp 

 edge; operculum corneous, fitting the mouth of the shell, 

 formed spirally by a few rapidly increasing whorls. It 

 includes one genus — Assiminia. 



Section II. — Gasteropoda Pulmonifera. 



So called from their moving on their stomachs, and 

 bearing lungs enabling them to breathe the atmosphere. 

 It embraces all the terrestrial mollusca, and the majority 

 of the gasteropods inhabiting fresh water. They are 

 furnished with pulmonary vessels distributed over the 

 walls of a sack or cavity, situated at the back of the 

 neck. The air admitted into this cavity oxygenizes the 

 blood. Those that inhabit the waters have to come 



