42 
B. Operculum subannular ; mantle and shell simple in front ; eyes 
sessile. 
Fam. 28. PxHorip. Foot compressed, used for jumping, not 
walking ; eyes sessile, on the outer side of the subulate tentacles ; 
operculum large, horny, subannular; muzzle conical, produced ; 
tongue ?; teeth ? Phorus, Onustus. 
C. Operculum annular, regular ; mantle with a siphon in front ; 
shell simple in front ; eyes pedicelled, near the outer side of the base 
of the subulate tentacles. : 
Fam. 29. AmpuLLARiAD#&. Gill with air-bag; central teeth 
acute; lateral subulate. 
a. Operculum horny ; canal exposed. Marissa, Pomus, Pomella, 
Lanistes. 
6. Operculum shelly ; canal not exposed. Asolene, Ampullaria. 
D. Operculum annular, regular ; mantle and shell simple in front ; 
eyes sessile, on the outer side of the base of the subulate tentacles. 
Fam. 30. Vivirartp&. ‘Teeth abnormal, laminar, longitudinal, 
ovate, apex recurved, toothed on each side the tip; inner lateral 
tooth broad (fig. 19). Viviparus, Paludomus, Bithinia. 
Fig. 19.—Viviparus. 
E. Operculum annular, regular, with an internal process ; mantle 
and shell simple in front; eyes sessile, far back behind the tentacles. 
Fam. 31. RissoeLLipz. Rostrum divided into two tentacular 
lobes in front; teeth 3°1°3 (Alder MSS.). Rissoella=Jeffreysia, 
Alder; Rissoina. 
_F. Operculum spiral (rarely wanting) ; mantle and shell generally 
simple, sometimes with a rudimentary siphon and a canal in front of 
shell; eyes sessile ; outer lateral teeth conical, curved. 
a. Hyes sessile, on outer side of tentacles. 
* Gills indistinct, in the form of series of vessels on the inner sur- 
face of the mantle. 
_ Fam. 32. CycLopuorip&. Operculum spiral; mouth of shell 
circular. Cyclophorus, Cyclotus, Pomatias, &c. 
Fam. 33. OLtigyrapa#. Operculum annular ; mouth of shell half 
ovate. Oligyra, Aleadia. 
