There are ten families in this order, all of which, as far as known, are widely or universally distributed. Some of them are found fossil, ranging back to the Carboniferous epoch. They are commonly termed Sea-slugs, and have either a thin small shell or none. We shall therefore simply enumerate the families, with the number of genera and species as given by Mr. Woodward.
Family 31.—TORNATELLIDÆ. (7 Genera, 62 Species living, 166 fossil.)
Family 32.—BULLIDÆ. (12 Genera, 168 Species living, 88 fossil.)
Family 33.—APHYSIADÆ. (8 Genera, 84 Species living, 4 fossil.)
Family 34.—PLEUROBRANCHIDÆ. (7 Genera, 28 Species living, 5 fossil.)
Family 35.—PHYLLIDIADÆ. (4 Genera, 14 Species living, 0 fossil.)
Family 36.—DORIDÆ. (23 Genera, 160 Species living, 0 fossil.)
Family 37.—TRITONIADÆ. (9 Genera, 38 Species living, 0 fossil.)
Family 38.—ÆOLIDÆ. (14 Genera, 101 Species living, 0 fossil.)
Family 39.—PHYLLYRHOIDÆ. (1 Genus, 6 Species living, 0 fossil.)