6 GASTEROPODA. 



Tribe III. — Helicid^. 



Animal — Head well developed^ with four cylindrical retractile 

 tentaculcBj the upper ones always prominent^ bearing on 

 their summits the eyes ; end of the tail tapering^ without 

 a gland; cavity of the lungs in the anterior part of the 

 body^ at the under part of the edge of which is the breath- 

 ing hole ; at the outer base of the right tentacle is the 

 orifice of the generative organs. Motion produced by the 

 successive expansions and contractions of the muscular 

 foot. 



Shell — Spiral, often turreted, rarely depressed or expanded, 

 never oper'culated, but this deficiency is made up by the 

 animal, just before hibernating, closing the orifice with a 

 membraneous epiphragm, which will always be found per- 

 forated for the admission of air This is by far the 

 largest tribe, including eleven genera — Vitrina, Zonites, 

 Helix, Bulimus, Pupa, Balsea, Clausilia, Zua, Azeca, 

 Achatina, Succinea. 



Tribe IV.— Limn^adje. 



Animal — With a conical spiral body; an elongated foot; short 

 muzzle with dilated lijjs, and two compressed tentacles of 

 considerable size, with the eyes placed near their outitr 

 base. The tongue is armed with numerous toothed den- 

 ticles. No operculum. 



Shell — Variously shaped^ spiral, turreted^ dextral or sinistral, 

 discoid and even patelliform. Pale, uniform coloured^ 

 and clothed with a hard olive or brownish periostraca. 

 It includes four genera — Physa, Planorbis, Limneeus, and 

 Ancylus. 



Tribe V. — Auriculid^. 



Animal — With a broad and ringed muzzle, on which are two 

 subtriangular and cylindrical tentaculcd, with eyes near 

 the inner bases. They have numerous hoolced teeth, 

 similar to the LimncBadcd. Foot nearly ovate, ivithout 

 operculum. 



Shell — Spiral ovate, with denticulated apertures. The septa 

 between the whorls of the shell at the extremity of the 

 body are imperfect. It includes two genera — Conovulus 

 a7id Carychium. 



