484 W. WILLIAMSON AND C. D. SOAR ON BRITISH HYDRACARINA : 



Sig Thor's exhaustive study of Lebertia has shown that the 

 genus can be resolved into two groups covering five sub-genera. 

 In the first group the skin is dotted over with fine pores, and 

 may be described as smooth, as it is without the papillae or 

 ridges found in the second group. Swimming hairs are always 

 present, though in rare cases these may be rudimentary. The 

 spines on the extensor surface of the first segment of the fourth 

 pair of legs also appear to lend themselves towards the dis- 

 crimination of the groups, as the second group may have from 

 five to ten, while the first group only has three or four, though 

 L. obscura forms a slight exception, as it has five or six. 



Two sub-genera, Lebertia (Sig Thor's Neolebertia) and Pilo- 

 lebertia, belong to the first or smooth-skinned group, and these 

 may be contrasted as follows : 



In sub -genus Lebertia the body is rather elongate. The 

 second pair of legs is without swimming hairs, while the third 

 and fourth pairs have only isolated ones on the fourth and fifth 

 segments. The number of these swimming hairs on each segment 

 varies may, indeed, even be wanting but does not exceed four. 

 It may be remarked here that in L. subtilis the swimming hairs 

 appear to be entirely wanting. The third and fourth segments 

 of the palpi are each fairly uniform throughout their length. 

 The third segment has on its inner surface five long bristles ; 

 three of these are distal, the middle one being fairly close to that 

 at the edge of the extensor surface. The fine pores on the flexor 

 surface of the fifth segment are not very distinct, while the few 

 small hairs on the extensor surface are entirely clustered at the 

 distal end, one or two isolated ones being placed rather farther 

 back. By Article 9 of the International Rules of Zoological 

 Nomenclature (1905) Neolebertia is suppressed in favour of 

 Lebertia as the name of the sub-genus. 



In sub-genus Pilolebertia the body varies from oval to nearly 

 circular in outline. The second, third and fourth pairs of legs 

 have numerous swimming hairs. Contrasted with sub-genus 

 Lebertia, the third segment of the palpi is more like an inverted 

 cone, while the fourth segment is rather curved. The inner 

 surface of the third segment has also five bristles ; three of these 

 are distal, the middle one being distant from the edge of the 

 extensor surface, not close to it as in Lebertia. The flexor 

 surface of the fourth segment has two distinct pores well 



