BY E. M. JOHNSTON, F.L.S. 



Genus Hydkobia. Hartmann. (290-293.) 

 Shell minute, ovate or elongated, smooth, sub-perforate ; spire 

 conic; whorls generally flat; apex acute; aperture 

 ovate; inner lip not thickened ; operculum corneous; 

 rostrum rather long ; tentacles somewhat tapering but 

 blunt at the extremity ; foot somewhat pointed behind. 

 Dist. — World- wide fresh and brackish waters. 



Genus Bithynella. ? Moquin-Tandon. (294-303.) 

 Shell minute, elongated-ovate, usually somewhat pupiform, 

 imperforate, or simply rimate ; apex obtuse ; aperture 

 oval or rounded ; peritreme continuous ; outer lip 

 sharp or slightly thickened ; operculum corneous, 

 nucleus moderately large, not very close to basal 

 margin. Tentacles tapering blunt at the tip ; foot 

 rather narrow, rounded behind ; very bifid. Fresh 

 water, ascending to 4,000 feet altitude in Tasmania. 

 Dist.— Europe, America, Australasia ? 



Genus Potamopyrgtjs. Stimpson. 

 Shell ovate-conic, imperforate ; apex acute ; whorls coronated 

 with spines : aperture ovate ; outer lip acute ; oper- 

 culum corneous ; rostrum moderate ; tentacles very 

 long, slender, tapering, and pointed ; eyes on very 

 prominent tubercles ; foot rather short, broadest in 

 front, and strongly auriculated. Dist. — Fresh water, 

 New Zealand, Tasmania, Cuba. 



Genus Potamopykgus. ? Hutton non Stimpson. (294-303.) 



Shell ovate-conic or oval, imperforate; body whorl more than 

 half the length of shell ; aperture ovate, the outer lip 

 acute ; peritreme continuous or discontinuous ; oper- 

 culum horny, sub-spiral, without any internal process. 

 Animal with the foot rather short, broadest, and 

 slightly expanded in front ; tentacles very long, 

 slender, tapering and pointed ; eyes on very prominent 

 tubercles. Dentition, median tooth trapezoidal ; inferior 

 margin more or less trilobate ; first lateral broad and 

 excavated in the middle, contracted into a long 

 peduncle, the denticles nearly equal ; second lateral 

 pointed at the inner extremity ; the shank broad 

 and thickened on its outer margin ; third lateral 

 with the inner extremity broad and rounded, con- 

 structed at its junction with the very broad shank 

 which is thickened on its outer margin. Number of 

 transverse rows of teeth, 55 to 69. Formula of the 

 denticles^" 9 - - 9 or 11 ; 20 to 23; 30 to 40. 



3 or 4—3 or 4 ' ' ' 



Mutton's new definition of Potamopyrgns would exactly 



