PYXIDICULA INVISITATA. 23 



Test discoid, often flattened on the dome, open 

 below, thin, chitinous, varying in colour from light 

 yellow to brown, the margin often colourless, punctate 

 with radial striations at the margin ; the open or 

 ventral side of the test provided with a circular ring 

 attached at a distance equal to its breadth from the 

 edge of the test, thus contracting the aperture ; plasma 

 greyish in colour, granular, containing numerous green 

 bodies and brown f ood -particles ; nucleus single; con- 

 tractile vesicle prominent, usually single. 



Diameter of test 25 p to 50 /x, usually about 40 /x. 



Habitat. Wet moss and moorland pools. 



ENGLAND. Cumberland, Westmorland, and Derby- 

 shire (Brown}. 



SCOTLAND. Inverness-shire, Elginshire, Ross-shire, 

 Argyllshire, and Perthshire (Brown),. 



The tst in section shows a thickening near the 

 margin where the marginal flanges join ; in some 

 cases they curve towards one another, whilst in others 

 the inner one projects inwards, thus contracting the 

 aperture of the test. 



It is probably common but is easily overlooked. 

 The nucleus is usually indistinct, but Averintzeff 

 figures it with a prominent, dark, central nucleole. 



The double marginal flange and its smaller size 

 distinguish this species from P. cymbalum Penard. 



3. Pyxidicula cymbalum Penard. 

 (Fig. 167.) 



Pyxidicula cymbalum 



PENARD Faune Rhiz. Leman (1902), pp. 417-418,4 figs. 

 EDMONDSON in "Ward & Whittle's Fresh- water Biology (1918), p. 222, 

 f. 292. 



Test moderately large, patelliform, chitinous, brown 

 in colour, punctate and furnished with a diaphanous 

 external margin or border ; nucleus single ; contractile 

 vacuole single ; pseudopodia not observed. 



Dimensions, 85 /x to 90 /A in diameter. 



