Fam. 15.—Phasianellidae. Shell not nacreous, without umbilicus, with prominent spire and polished surface. Phasianella.
Fam. 16.—Umboniidae. Shell flattened, not umbilicated, generally smooth; operculum horny. Umbonium. Isanda.
Fam. 17.—Neritopsidae. Shell semi-globular, with short spire; operculum calcareous, not spiral. Neritopsis. Naticopsis, extinct.
Fam. 18.—Macluritidae. Extinct, Cambrian and Silurian.
Fam. 19.—Neritidae. Shell with very low spire, without umbilicus, internal partitions frequently absorbed; a single ctenidium; a cephalic penis present. Nerita, marine. Neritina, freshwater, British. Septaria, shell boat-shaped.
Fam. 20.—Titiscaniidae. Without shell and operculum, but with pallial cavity and ctenidium. Titiscania, Pacific.
Fam. 21.—Helicinidae. No ctenidium, but a pulmonary cavity; heart with a single auricle, not traversed by the rectum. Helicina. Eutrochatella. Stoastoma. Bourceria.
Fam. 22.—Hydrocenidae. No ctenidium, but a pulmonary cavity; operculum with an apophysis. Hydrocena, Dalmatia.
Fam. 23.—Proserpinidae. No operculum. Proserpina, Central America.
Order 2. Pectinibranchia.—In this order there is no longer any trace of bilateral symmetry in the circulatory, respiratory and excretory organs, the topographically right half of the pallial complex having completely disappeared, except the right kidney, which is represented by the genital duct. There is usually a penis in the male. The ctenidium is monopectinate and attached to the mantle along its whole length, except in Adeorbis and Valvata; in the latter alone it is bipectinate. There is a single well-developed, often pectinated osphradium. The eye is always a closed vesicle, and the internal cornea is extensive. In the radula there is a single central tooth or none.