PSEUDOCHLAMYS PATELLA. 181 
body to the interior of the test, the latter appearing to 
be fully occupied, when the discoid figure is examined, 
except a narrow hyaline margin. Usually the animal 
begins an active movement by raising the margin of 
the test on the opposite side; the movement will be 
continued until the opposite margins approach and 
the figure becomes roughly-speaking semi-circular ; or 
the margins will overlap, and the animal will remain 
thus curiously rolled together for some time, its 
general form being comparable to that of the chrysalis 
of a lepidopterous insect. 
The discoid form admits of the endoplasm being 
readily studied. Four or more contractile vacuoles 
may usually be seen, pulsating languidly, on the outer 
margin of the plasma-body ; and a central nucleus is 
also visible, though not conspicuous. 
The brown colour of the test and its rigidity are dis- 
tinctive of old individuals. Empty tests may at an 
time be met with where the species is abundant; they 
are seen to vary little, if at all, in size or structure. 
They differ widely in appearance from the tests of the 
Arcelle, being very much smaller, simple, and concave, 
usually of a deep brown, and sometimes punctated 
about the centre, the colour gradually shading off 
towards the margin, which is rarely found inverted. 
Although undoubtedly distinct, the affinities of 
Pseudochlamys are with Arcella. We have found the 
organism most abundant in spring, amongst floating 
Hypna in ponds. Later in the year it seems to 
disappear. 
Genus 17. CENTROPYXIS Stein, 1859. 
Arcella (pars) Enxrenperc in Abh. K. Akad. Wiss. Berlin, 
1830 (1832), p. 40. 
Arcellina Canton in Ann. Nat. Hist. (1) IX (1842), p. 362. 
Difflugia (pars) Perry in Mitth. nat. Ges. Bern, 1849, p. 45. 
Centropyxis Stein in Abh. K. Bohm. Ges. Wiss. X (1859), 
Berichte, p. 43. 
Echinopywxis Cuaparpg and Lacumann Etudes Inf. et. Rhiz. 
2 (1859), p. 447. 
