Order—ISOPODA.
Tribe—ONISCOIDA.

Family I.—Ligiidae.Family III.—Oniscidæ.
Ligia.Oniscus.
Ligidium.Philoscia.
Platyarthrus.
Porcellio.
Metoponorthus.
Cylisticus.
Family II.—Trichoniscidæ.Family IV.—Armadillidiiæ.
Trichoniscus.Armadillidium.
Trichoniscoides.
Haplophthalmus.

All the genera described by Professor Sars are represented in the British Islands.

Below is a scheme of classification and synopsis of the characters of British genera of woodlice which we have compiled in order to render easy the determination of the genus to which any particular specimen may belong.

SCHEME OF CLASSIFICATION AND SYNOPSIS OF GENERIC CHARACTERS.
Order—ISOPODA.
Tribe—ONISCOIDA.
Section I.—LIGIÆ.
The Two Divisions of the Tail Appendages alike in Shape.

(A.)—Flagellum with 10 or more joints; tail appendages
wholly visible; head without lateral lobes 		Ligiidae.
(1.)—Abdomen broad; body large; habitat,
the sea-shore 		Ligia.
(2.)—Abdomen narrow; habitat, wet moss Ligidium.
(B.)—Flagellum with less than 10 joints; head with
small lateral lobes, tail appendages partly
covered 		Trichoniscidæ.
(3.)—Abdomen narrow; eyes compound;
flagellum usually with more than 3
joints 		Trichoniscus.
(4.)—Abdomen narrow; eyes simple or wanting;
flagellum with 4 joints 		Trichoniscoides.
(5.)—Abdomen broad (comparatively); eyes
simple; back with longitudinal
ridges; flagellum with 3 joints 		Haplophthalmus.

Section II.—ONISCI.
The Outer Divisions of the Tail Appendages Broader than the Inner.

(A.)—Tail appendages projecting when the animal is
walking 		Oniscidæ.
(a.)—Unable to roll up into a complete ball.
(6.)—Flagellum with 3 joints; abdomen
broad; head, with lateral lobes 		Oniscus.
(7.)—Flagellum with 3 joints; abdomen
narrow; head without lateral lobes 		Philoscia.
(8.)—Flagellum with 1 joint; eyes wanting;
abdomen broad; habitat, ant's nests 		Platyarthrus.
(9.)—Flagellum with 2 joints; abdomen
broad; frontal lobe projecting 		Porcellio
(10.)—Flagellum with 2 joints; abdomen
narrow 		Metoponorthus.
(b.)—Able to roll up into a complete ball.
(11.)—Flagellum with 2 joints; antennae
folded together over the thorax when
the animal is rolled up into a ball 		Cylisticus.
(B.)—Tail appendages not projecting when the animal
walking 		Armadillidiidæ.
(12.)—Flagellum with 2 joints; antennae
hidden or carried at the sides of the
head when the animal is rolled up
into a ball 		Armadillidium.

British Species.—Naturalists in this country paid little attention to the recognition or description of Woodlice, until the latter half of the nineteenth century.