On Pyxidicula invisitata and Hedriocystis spinifera. 171 



Sub-Class EHIZOPODA. 



Order Conchulina. 



Pyxidicula invisitata Averintzeff. 



Test watch-glass shaped, often flattened dorsally, delicate, 

 chitinoid ; middle region (disc) varying in colour from light yellow 

 to dark brown ; marginal region (flange) always lighter in colour 

 and frequently almost colourless ; middle region in surface-view 

 punctate ; marginal flange showing radial striations. 



In transverse section the margin of the test appears to be 

 double, owing to the flange being inserted above the edge of the 

 disc, which is turned inwards slightly, thus contracting the 

 mouth-aperture. Protoplasm greyish in colour, granular, contain- 

 ing numerous green and brown bodies (food-particles) ; nucleus 

 indistinct (Averintzeff figures it with dark prominent central 

 nucleolus) ; contractile vacuole usually single, prominent. 



Diameter of test 25-50 /a, most often about 40 /u. (Averintzeff 

 gives 45-50 /i). 



Habitat. — Wet moss and moorland pools. 



Distribution. — Derbyshire, Westmorland, Cumberland, Inver- 

 ness-shire, Argyllshire, Perthshire, Elginshire, Eoss-shire. 



It appears to be common, but is easily overlooked. 



The margin of the test is distinctive. In dorsal view it appears 

 as a transparent flange, as in P. cyrnbalum Penard. In optical 

 section the edge of the darker disc shows a marginal thickening 

 triangular in section, and from the two lower corners of the triangle 

 arise an outer and an inner flange, the outer one being that seen 

 from above, and the inner one being, perhaps, merely a continuation 

 oral-wards of the disc. Some tests show the outer flange curved 

 ventrally inwards, while others show both flanges curved together. 



In surface-view the animal might easily be mistaken for a small 

 specimen of P. cyrnbalum. 



Amongst a quantity of damp moss from the Isle of May, 

 Pirth of Forth, and reported on in the " Scottish Naturalist " for 

 1912 (pp. 108-14), a small number of individuals of a minute 

 Heliozoon related to Hedriocystis reticulata. Penard (" Les Helio- 

 zoaires d'eau douce," 1904, p. 284) were discovered. They present 

 sufficiently important differences from that species, however, to 

 warrant regarding them as belonging to a new species. 



N 2 



