6 INTRODUCTIO. 
a Cl. H. et A. Adams Societati zoologicae Londinensi pro- 
posita et 1855 in opere Eorundem: 'The genera of recent Mol- 
lusca; IL p. 236—250 systematice redacta characteres fami- 
liae, subfamiliarum, generum et subgenerum sequentes exhibet : 
Fam. Ellobiidae. Lingual membrane broad and elongated; 
teeth numerous, in slightly-bent, cross series; central tooth 
equilateral, narrow, tricuspid; lateral teeth rather inequilateral, 
diminishing in size towards the outer edge. Head ending in 
a snout; mouth with a horny, lunate upper jaw, and with 
two dilated buccal lobes, united above, separate below; ten- 
tacles subeylindrical, contractile; eyes sessile at the inner sides 
of their bases.  Mantle closed, with a thickened margin; re- 
spiratory orifice posterior, on the right side. Sexes united. 
— Shell spiral, covered with a horny epidermis; aperture 
elongate, with strong folds on the inner lip; outer lip often 
dentate. — Animal lacustrine, usually frequenting salt marshes. 
I. Sub-fam. Ellobiinae: Animal terrestrial, living chiefly 
on the land. "Tentacles developed, eyes at their inner bases. 
— Shell with the inner lip plicate; outer lip thickened or 
expanded. 
1l. Genus Ellobiun Bolten. Mantle-margin  thickened; 
foot simple. — Shell oblong-oval, covered with a brown 
epidermis; spire short, obtuse; aperture narrow, elongated; 
inner lip with two or three large anterior plaits; outer lip 
thickened internally. 
2. Genus Cassidula Fér. Mantle-margin reflexed on the 
inner edge of the outer lip; foot bifid posteriorly. — Shell 
oval, sub-conie, solid; spire short, obtuse; aperture narrow, 
longitudinal; inner hip plicate anteriorly and reflected; outer 
hp reflexed, with an internal, marginal callus or ledge sinua- 
ted posteriorly. 
Sub-gen. Sarnia H. et A. Adams. Shell ovato-cylindrical; 
spire obtuse, whorls transversely striated; aperture linear; inner 
hp with two anterior plaits; outer lip thickened internally, po- 
steriorly sinuated.  (Siona in Proc. Zool. Soc. 1854. p. 32.) 
3. Genus Pythia Bolt. Foot ovate, simple posteriorly. —. 
Shell ovate; spire acute, whorls compressed, with a varix on 
each side extending as far as the apex; aperture narrow, 
ringent; inner lip strongly plicate; outer lip sub -reflexed, 
dentate internally. 
4. Genus Plecotrema H.et A. Adams. Shell ovate, solid, 
umbilicated; spire elevated, conie, whorls transversely groov- 
ed; aperture contracted, oblong; inner lip with three plaits, 
the middle plait bifid; outer lip with a marginal varix, inter- 
nally bidentate. 
