BRITISH HYDRACHNIDyE. 275 



of the glass tanks in which I keep them in the same manner as I 

 described in Nescea. 



Not having a male of this species, I have drawn the ventral side 

 of Pmia affinis $ (Koch), Fig. 6, thinking it would be interesting 

 to show the difference in the structure of the epimeral plates in the 

 male and the female ; it will be noticed that the two posterior pair 

 are very large, and nearly touch in the centre. This Hydrachnid 

 is not so large or so beautiful as P. ovata ; it is quite an ellipse in 

 shape, pale blue in front, and gradually turning to yellow ; the 

 posterior margin is quite yellow; it has a yellow cross on the back 

 surrounded with dark brown markings. Both male and female 

 are the same colour. 



In Fig. 7 I give a drawing of the nymph stage of Fiona affinis, 

 and which, from the position of the epimera, I take to be the 

 nymph stage of a female. How long it remains in this stage I 

 cannot say. 1 have kept it eight months in a tube and it still 

 looks in good condition, it has plenty of colour (it is the same 

 colour as the adults), and swims about vigorously. I have exam- 

 ined it at intervals, but can see no change whatever ; in this stage 

 there are only two genitals on each side in a special plate. The 

 nymph of Fiona can easily be recognised from the nymph of 

 Nescea by the last pair of epimeras being extended backwards 

 to a point. 



EXPLANATION OF PLATE XIL 



Fig. 1. — Dorsal view of Fiona ovata, ^ • 



2. —Palpus of same, showing the peg on the fourth joint. 



3. — Genital area of same, showing the position of genital suckers. 



4.— Mandible. 



5. — Ventral view of same. 



6. — Ventral view of Fiona affinis, ^ , 



7. — Ventral view of Fiona affinis, nymph. 



8. — First leg of Fiona ovata, ^ . 



9. — Fourth leg of ditto. 



