36 A EE VISION OF THE BEITISH SPECIES OF 



similar but smaller organ in the same position on the right foot. 

 Inner branch of the right fifth foot very small, pyriform, one- 

 jointed, acuminate : terminal claw of the outer branch long, 

 slender, subsigmoid. Length 1*1 mm. 



I took a few specimens only of this, which appears to be a 

 quite distinct species, in Goat Water, a tarn lying at a con- 

 siderable elevation on the side of Coniston Old Man. More 

 recently (1891), Mr. Scott has sent me specimens from Loch 

 Harray, Orkney, where he found it abundantly. 



6. Diaptomus serricornis, Lilljeborg (PL IX., figs. 3-10). 



1888. Diaptomus serricornis, Lilljeborg (53), p. 157. 



1888. „ Wierzejshii, Eichard (52), p. 45. 



1889. ,, ,, De Guerne & Eichard (62), 



p. 35, pi. II., figs. 10, 22; 



pi. III., fig. 5. 



1889. ,, serricornis, De Guerne & Eichard (62), p. 37, 



pi. L, figs. 20, 21, 30. 



Smaller and more slender than D. Castor, but larger than D. 

 gracilis. 



Female. — The posterior thoracic angles, seen from above, are 

 rounded, moderately prominent, and furnished with (usually 

 two) small mucrones. The first abdominal segment is broad at 

 the base, and expanded laterally into more or less acuminate 

 triangular processes. Anterior antennae twenty- five- jointed, 

 and reaching as far back as beyond the middle of the abdomen. 

 Mouth-organs and first four pairs of feet as usual in the genus. 

 Angle of the basal joint of the posterior maxilliped rounded, 

 slightly crenulated, and bearing about six small marginal hairs 

 but no long seta. Inner branch of the fifth foot small, one- 

 jointed, simple, cylindrical, about half as long as the basal joint 

 of the outer branch, bearing at its apex two very minute cilia ; 

 outer branch three- jointed; first joint simple, cylindrical; se- 

 cond, large at the base, but contracted distally, forming a stout 

 curved claw, which is minutely ciliated on its concave margin ; 

 last joint extremely small, and having two apical setae, one 

 large and one small, neither of which reaches nearly as far as 

 the apex of the second joint. 



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