96 



A KEVISION OF THE BRITISH SPECIES OP 



This species seems to be universally distributed througb. the 

 British Islands, but is not found in waters smaller in extent 

 than lakes or very large ponds, frequently in such situations 

 occurring in vast numbers. By the deep-water net in depths of 

 50-80 fathoms it is often taken in abundance, and in one in- 

 stance, at least (Talkin Tarn, Cumberland), I have seen the 

 net come up from a depth of six or eight feet below the sur- 

 face with a dense mass consisting almost entirely of D. gracilis. 

 It is also taken quite commonly at the surface, but scarcely ever, 

 in my experience, among weeds, though it occurs quite close to 

 the shore on the stony margins of lakes and tarns. B. gracilis 

 has been recorded as occurring in JS'orway (Gr. 0. Sars), Sweden 

 (Lilljeborg), Finland (Nordquist), and by various authors in 

 Germany, Switzerland, and Italy. 



That a species so widely distributed, and occurring often in 

 numbers so enormous, should show a marked tendency to varia- 

 tion, is only what one would expect ; and while hesitating 

 to express an opinion different from that which has been arrived 

 at by observers so careful and accurate as Sars, Lilljeborg, 

 De Guerne & Eichard, and Poppe, I cannot help believing that 

 B. gracilis and B. graciloides certainly, and some other so-called 

 species, very possibly, ought to be considered as referable to one 

 only. I will briefly state my reasons for this view. The charac- 

 ters which are chiefly relied on to distinguish B. graciloides, 

 Lilljeborg, from B. gracilis, Sars, may be stated as follows : — 





D. gracilis, Sars. 



D. graciloides, liiWi^hovg. 



Inner branch of the fifth 

 foot of t\\Q female 



very short 



long 



Inner branch of the fifth 

 foot in the male (right 

 side) 



Terminal claw of fifth 

 foot in the male 



long 



simply falcate 



short 

 sigmoid 



Last joint of external 

 branch of the left, fifth 

 foot of male 



bearing on Its internal 

 margin a process with 

 an apical bunch of 

 hairs 



bearing a simple ciliated 



seta 



Antepenultimate joint of 

 riqht male antenna 



having an apical hook- 

 like prolongation 



simple — not produced 



