504 W. WILLIAMSON AND C. D. SOAR ON BRITISH HYDRACARINA I 



Lebertia obscura Sig Thor. 

 (Sub-gen. Pilolebertia.) 



1900. Lebertia porosa var. obscura Sig Thor. Nyt. Mag. Naturv., 



xxxviii. 273, pi. x. figs. 3 and 4. 

 1902. L. obscura Sig. Thor in Arb. Inst. Wien, xiv. (2) 11, pi. i. 



fig. 9- 

 1906. Sig Thor, Zool. Anz., xxix. 780, figs. 47-48-54. 



Lebertia obscura was, by reason of its closeness to L. porosa, 

 first considered by Thor to be only a variety of that species, but 

 his exhaustive investigations on the genus led him to elevate 

 obscura to the rank of a distinct species. When obscura and 

 porosa are contrasted, it will be noted that the former is some- 

 what the larger of the two, ranging from 1*5 mm. to 2 "5 mm. in 

 length, and in general it appears to be of a more robust build and 

 somewhat darker colour. 



The palpi measure up to about 0'65 mm. in length, and viewed 

 ventrally are scarcely so stout as the first pair of legs. Compared 

 with porosa the long bristle on the flexor surface of the second 

 segment is relatively shorter and situated rather more distally, 

 the third segment is shorter in proportion, while the fourth is 

 broader and straighter, with the two pores on the flexor surface 

 frequently situated close together. As a general rule the fifth 

 segment is shorter and more blunted. 



The legs are thick and strong. The first pair measures about 

 0*96 mm., the second pair about 1*36 mm., the third pair about 

 1*70 mm., and the fourth pair about 1*92 mm. in length. The 

 fourth pair is to be noted as possessing in the fifth and sixth 

 segments a larger number of spines and swimming hairs than in 

 the closely allied species, and also what Thor deems a characteristic, 

 the possession of 5 or 6 spines, 3 or 4 of these being distal, on the 

 extensor surface of the first segment, instead of the 3 (more rarely 

 4), 2 of them being distal, generally associated with other Pilo- 

 lebertia species. 



L, obscura does not appear to be a widely distributed species, 

 as so far it has only been reported from Norway, Scotland and 

 England. 



