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 food-particles ; nucleus single; con- 
tractile vesicle prominent, usually single. 
Diameter of test 25 pw to 50 w, usually about 40 p. 
Habitat.— Wet moss and moorland pools. 
EneLtanp.—Cumberland, Westmorland, and Derby- 
shire (Brown). ’ 
Scottanp.—Inverness-shire, Elginshire, Ross-shire, 
Argyllshire, and Perthshire (Brown),. 
The test 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. cymbalwim Penard. 
3. Pyxidicula cymbalum Penard. 
(Fig. 167.) 
Pyxidicula cymbalum 
PENARD Faune Rhiz. Léman (1902), pp. 417-418, 4 figs. 
Epmonpson 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 pw to 90 w in diameter. 
