We are able to include this species, as a specimen was found by Mr. Webb at Eton Wick in the summer of 1899. It is one of a number of species which the Rev. Canon Norman (49, p. 18) suggested as likely to be British. It is the only representative of its genus, which does not differ in any very important characters from the others in the family. The narrow elongated body will serve to separate it from _Trichoniscus vividus_ and _Trichoniscus roseus_, but on account of its size, which is much the same as that of _Trichoniscus pusillus_ and the two British species of _Haplophthalmus_, it will be advisable to give some further points of distinction. From the first its white colour will serve to differentiate it; the other two lack the narrow abdomen seen in _Trichoniscoides albidus_. Moreover, not one of the three shows the serrations on the side plates which characterise the species under consideration. _Platyarthrus hoffmannseggii_ is small and white and the edges of its side plates are toothed, but it is oval in shape, possesses no eyes, and its stout antennæ have but a single joint to the flagellum instead of four. On the Continent this species has been found in rich soil. _BRITISH LOCALITIES:--_ _England_: Eton; (Stebbing, 71a): Sunderland; (Brady, 50a). _FOREIGN DISTRIBUTION:--_ _Europe_: France; Wimereux and Lyons, Forêt (25): Norway; Denmark; (59). _Genus_--=HAPLOPHTHALMUS= Schöbl, 1850 (66), p. 449. _Abdomen broad (comparatively); eyes simple; flagellum with three joints; back with longitudinal ridges._ The body of _Haplophthalmus_ is long in proportion to its width, but there is no abrupt decrease in the breadth of the abdomen as seen in _Trichoniscus_ and _Trichoniscoides_. The eyes are simple as in the latter genus and the lateral lobes of the head are rather large, while the side plates of the body are well separated. =Haplophthalmus mengii= Zaddach. PLATE VII. 1844 _Itea mengii_ Zaddach (77), p. 16. 1860 _Haplophthalmus elegans_ Schöbl (66), p. 449. 1885 _Haplophthalmus mengii_ Budde-Lund (8), p. 250. 1898 _Haplophthalmus mengii_ Sars (59), p. 167, pl. LXXIV., fig. 1. The Rev. Canon Norman discovered two specimens of this species in Ireland in June, 1900 (50); in the previous year one of us (Mr. Webb) found a single example at Eton Wick. The main differences between the members of this genus and their allies are set forth in the generic description and incidentally elsewhere, so we shall content ourselves with giving the distinctive points of the two British species. _Haplophthalmus mengii_ has a number of raised longitudinal ribs on each segment of the thorax, the outer ridges being somewhat broken. There are also two prominent ribs upon the third segment of the abdomen. [Illustration: FIG. 41.--FLAGELLUM AND LAST PEDUNCULAR JOINT OF THE ANTENNA OF _Haplophthalmus mengii_.] _BRITISH LOCALITIES:--_ _England_: Eton; (Stebbing, 71a): Sunderland; (Brady, 50a).