THE BRITISH WOODLICE. 55
Classification.—The various genera of woodlice are con-
nected together so closely, by intermediate forms, that their
division into families is to a very great extent, arbitrary. Bate
and Westwood described but a single family Oniscidae (1),
though they distinguished two sub-families :—Ligiinae, which
included the forms with many joints to the flagellum of the
antenna, and Oniscinae, which contained the rest.
Since then the pill-woodlice have been thought by some, to
be sufficiently different from the other genera to warrant their
separation, and three families namely, Ligiidae, Oniscidae, and
Armadiilidae have been recognized, as for instance by Dr.
Scharff (63).
A fourth family—Trichoniscidae—has been added by
Professor G. O. Sars, who in his Crustacea of Norway (59)
alludes to the division of the tribe into the sections Ligiae and
Onisci and has adopted the following classification :—
Order—ISOPODA.
Tribe—ONISCOIDA.
Family I.—Ligiidae. Family III.—Oniscidae.
Ligia. Oniscus.
Ligidium. Philoscia.
Platyarthrus.
Porcellio.
Metoponorthus.
Family II.—Trichoniscidae. Cylisticus
Trichoniscus.
Trichoniscoides. Family IV.—Armadillidiiae.
Haplophthalmus. Armadillidium.
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.