Family 40.—ELYSIADÆ. (5 Genera, 13 Species living, 0 fossil.)
Order IV.—NUCLEO-BRANCHIATA.
These are oceanic, swimming molluscs, of a delicate texture. They are found in all warm seas, and range back to the Lower Silurian epoch. There are only two families.
Family 41.—FIROLIDÆ. (2 Genera, 33 Species living, 1 fossil.)
Family 42.—ATLANTIDÆ. (5 Genera, 22 Species living, 159 fossil.)
Class.—PTEROPODA.
These are swimming, oceanic mollusca, inhabiting both Arctic, Temperate, and Tropical seas. The three families have each a wide distribution in all the great oceans. They range back to the Silurian period.
Family 1.—HYALEIDÆ. (9 Genera, 52 Species living, 95 fossil.)
Family 2.—LIMACINIDÆ. (4 Genera, 19 Species living, 0 fossil.)
Family 3.—CLIONIDÆ. (4 Genera, 14 Species living, 0 fossil.)