BLAINVILLE. 543 



tained-in a cavity of which the upper wall is not pro- 

 longed into a canal, but there is sometimes an ap- 

 pendage or inferior lobe to do its office. Shell very 

 variable in form, the aperture entire and operculatc ; 

 the operculum horny or calcareous, and proportionable 

 to the aperture. 



Family 1. Goniostomata = Trochus, Lin. 

 ,, 2. Cricostomata = Turbo, Lin. 

 ,, 3. Ellipsostomata. Shell vai-iable in form, 

 ordinarily smooth, aperture oval longitudinal or 

 sometimes transverse, closed with a horny or 

 calcareous operculum. Ex. Melania, Rissoa, 

 Phasianella, Ampullaria, Helicina, &c. 

 ,, 4. Hemicyclostoma = Nerita, Lin. 

 „ 5. OxYSTOMA. Genus : lanthina. 



Subclass IL P. MONOICA. 



Sexes distinct, but the same individual both male and female. 

 Section I. Respiratory vrgans and shell {ivhen it exists) non- 

 symmetrical. 



Order I. Pulmobranchiata. Organs of respiration re- 

 tiform or aerial, clothing the floor of the cavity, placed 

 obliquely from left to right on the origin of the back 

 of the animal, and communicating with the circum- 

 fluent fluid by a small round orifice, pierced on the 

 right side of the swollen margin of the mantle. 

 Family L Limnacea. Ex. Limnaea, Planorbis. 



„ 2. Auriculacea. Genera : Pedipes, Auri- 

 cula, Pyramidella. 

 „ 3. LiMACiNEA. * Ex. Bulimus, Pupa, Helix. 

 ** Testacella, Limax. 

 Order II. Chismobranchiata. Organs of respiration 

 aquatic, or branchial pectinate, situated at the anterior 

 part of the back in a large cavity communicating with 

 the circumfluent fluid by a wide oblique and anterior 

 fissure. Mouth without a tooth, but furnished in- 

 feriorly with a long lingual ribbon. Shell or none, or 

 internal or external, very depressed, with a very large 

 entire aperture and no columella. 



The genera are Coriocella, Sigaretus, Cryptostoma, 

 Oxinoe, Stomatella, Vclutina. 

 Order III. Monopleurobranchiata. Organs of respira- 

 tion branchial, situated on the right side of the body 

 and covered more or less by the operculiform mantle, 

 in which there is often a Sliell, plane, or more or less 



