In _Trichoniscus_ the flagellum may have from seven to four (rarely three) joints. As in _Trichoniscoides_ and _Haplophthalmus_ there are lateral lobes to the head, though these are not very pronounced; the body is also of small size, the abdomen narrow with both divisions of the tail appendages equally so, and almost of the same length though slightly covered by the last segment. The compound eyes distinguish _Trichoniscus_ from the two genera named, and from _Platyarthrus_, while its small size and the character of its tail-parts mark it out from all others. =Trichoniscus pusillus= Brandt. Plate III. 1833 _Trichoniscus pusillus_ Brandt (3), p. 174, pl. IV., fig. 9. 1838 _Itea riparia_ Koch (34), part 22, pl. XVII. 1844 _Itea lævis_ Zaddach (77), p. 16. 1857 _Philougria celer_ Kinahan (32), p. 281, pl. XXII., figs. 1-4. 1858 _Philougria riparia_ Kinahan (33), pp. 191 and 198, pl. XXIII., fig. 1. 1868 _Philougria riparia_ Bate and Westwood (1), p. 456. 1898 _Trichoniscus pusillus_ Sars (59), p. 161, pl. LXXII., fig. 1. This tiny species is found commonly amongst the roots of the herbage in very moist places. It presents a horny translucent appearance and is of a reddish brown colour. It runs with considerable speed, and when it is moving, the white irregular lines with which it is beset are not evident. _Trichoniscus pusillus_ is very much like _Trichoniscus vividus_ in colour but the latter species is nearly twice as big and has from five to seven joints to the flagellum, while the former has never more than four. _Trichoniscus roseus_ is also much larger and its bright red colour (which it loses, however, when preserved in alcohol) is another means of distinguishing it from the species under consideration. Professor Sars in his _Crustacea of Norway_ (p. 162) describes from Christiania, under the name of _Trichoniscus pygmæus_, a still smaller species. As this may possibly be discovered in this country a brief comparison between it and _Trichoniscus pusillus_ may be of value. The former reaches a length of but two millimetres; it is "whitish, semi-pellucid with a few light brown pigmentary ramifications across the segments and a double row of irregular opaque patches along the middle of its back" (p. 163). Its body is covered with minute tubercles and there are only three joints to the flagellum; its movements are by no means rapid. [Illustration: FIG. 37.--FLAGELLUM AND LAST PEDUNCULAR JOINT OF THE ANTENNA OF _Trichoniscus pusillus_.] The body of _Trichoniscus pusillus_ is smooth and polished. It has four joints to the flagellum--Dr. Scharff (63) says three or four--and it moves quickly. _BRITISH LOCALITIES:--_