Fam. 42. Tritonidae.—Foot short, narrow; siphon short, not prominent; radula allied to that of Cassididae; shell thick, varicose; outer lip inflected and thickened, canal long, periostracum often thick and hairy, operculum corneous, nucleus terminal or sub-marginal. Cretaceous ——. Genera: Triton (Fig. [191], p. 275; subg., Epidromus, Plesiotriton, Simpulum, Ranularia, Argobuccinum); Persona, aperture toothed, narrow; columella reflected upon the last whorl; Ranella, shell dorso-ventrally compressed, generally with two continuous lateral varices, posterior canal present.
The position of the following four families is doubtful:—
Fam. 43. Oocorythidae.—Siphon short, foot broad, eyes absent, radula taenioglossate; shell buccinoid or cassidiform, operculum corneous, spiral. Recent. Single genus, Oocorys.
Fam. 44. Subulitidae.—Shell elongate, fusiform, smooth; suture shallow, base truncate or rounded, aperture channelled or notched. Ordovician to Trias. Genera: Subulites, Fusispira, Euchrysallis.
Fig. 278.—Pirula Dussumieri Val., Philippines. × ½.
Fam. 45. Seguenziidae.—Radula taenioglossate, shell trochiform, aperture channelled, columella twisted, operculum multispiral, nucleus central. Pliocene ——. Single genus, Seguenzia.
Fam. 46. Choristidae.—Anterior tentacles united by a frontal veil, posterior simple; eyes absent, foot with tentaculae before and behind; three central teeth, outer marginal with a basal plate; shell helicoid, suture deep, peristome continuous, operculum corneous, paucispiral. Pliocene ——. Single genus, Choristes.
Section II. Heteropoda.—Foot fin-shaped, not flat.
The Heteropoda are free-swimming Mollusca, being, like the Pteropoda, Gasteropoda modified to suit their pelagic environment. Their nervous system is streptoneurous, and they are therefore probably derived from the Prosobranchiata, but they are highly specialised forms. Pelseneer considers them far more widely removed from the Streptoneura than the Pteropoda are from the Euthyneura. They swim on the surface “upside down,” i.e. with the ventral side uppermost.