From the data presented in the previous pages it is evident that the large planorboid snails are divisible into three genera. Each has a form of genitalia peculiar to itself and different from any of the others. The radula does not afford characteristics of generic value, but may be used for subgeneric distinctions. The shells do not affprd tangible generic characters, but may also be used for subgeneric distinction. The groups as now understood may be distinguished as follows:—
- 1Vergic sac appearing as an enlargement of the vas deferens, very small and narrow, about one-sixth as long as the preputial sac; verge very amall, not evertible, but discharging the spermatozoa into a channel on an appendage connecting with the spermatophore gland, the appendage performing the fnnction of a verge; no spermatophore duct; anterior and posterior retractor muscles attached to preputial sac…Planorbis.
- 2Vergic sac very large, wide, one-third of to as long as preputial sac; verge evertible, large, discharging spermatozoa directly into feniale opening; spermatophoro gland forming a cup-shaped body with many foldn in the cavity; a spermatophore duct of varying length connecting spermatophore gland with vcrgic sac, into which it opons just above a muscular diaphragm separating the two sacs; one retractor muscle, which may be split into two or morc branches, attached to the preputial sac…Helelisoma.
- 3Vergic sac very long and narrow, as long as the sac; verge long and slender, evertihle; no spermatophore duct; spermatophore gland represented by heavy pads on the walls of the preputial sac…Indoplanorbis.The genus Helisoma is divisible into four subgenera, three of which may be diagnosed anatomically:—
- 4Vergic sac nearly as wide as preputial sac, with but a slight constriction between the two saes; duet very short, about as long as preputial sac; lateral teeth with mesocones sharply pointed…Helisoma.
- 5Vergic sac as long as preputial sac, pyriform, much narrowed at the constriction between the two saes; duct very long, one and a half to more than twiee as long as preputisl sac; spormatophore gland with willls much thicker on one side; gland wider than high; mesocone of lateral teeth very wide and blunt; aperture of shell campanulato…Planorbella.
- 6Vergic sac as in 2; spermntophorc gland with walks thickened on all sides; shell with apcrture not campsnulate…Pierosoma.
- 7Genitalia unknown; shell vertically compressed, whorls smaller and more numerous, apertnre very oblique…Planorbina.
A systematic arrangement of the genera and subgenera will appear as follows:—
Genus Planorbis Miiller, 1774. Type, Helix cornea Linn. Mainly European in distribution.
Genus Helisoma Swainson, 1840. Type, Planorbis bicarinutus Sowb. North and South America and West Indies in distribution.
Subgenus Helisom Swainson, typical. Type, Planorbis bicarinutus Sowb. North America in distribution.
Subgenus Planmhella Hald., 1842. Type, Planorhis campanulutus Say. United States and Canada in distribution.
Subgenus Pierosoma Dall, 1905. Type, Planorbis trivolvis Say. North and Central America in distribution.
Subgenus Planorbina Hald., 1842. Type, Planorbis olivaceus Spix.
South America, Central America, Mexico, southern United States, and West Indies in distribution.
Genus Indoplanmbis Annandale & Prashad, 1920. Type, Planorbis exustus Desh. Indo-China in distribution.