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 (i), 

 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 

 Armadillidae 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. 



Ligia. 

 Ligidium. 



Family II. — Trichoniscid^. 



Trichoniscus. 



Triclwniscoides. 



Haploplithalmus. 



Family III. — Oniscid^;. 

 Oniscus. 

 Philoscia. 

 Platyarthnis. 

 Porcellio. 

 Metoponorthus. 

 Cyl/sticus 



Family IV.- 



-Armadillidii/e. 



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. 



