122 BRITISH FRESHWATER RHIZOPODA. 
Test gibbous, varying structurally from the type, 
and from var. «ngulosa, in having the surface pitted 
with circular depressions, which occur in regular 
series, are of uniform dimensions, and appear in trans- 
verse view as small concavities producing a broken 
outline. Protoplasm, nuclei, and contractile vesicles 
as in the type. 
Dimensions: Average diameter of pond-form 45-50 p; 
sometimes much larger. At Park Bridge, Lancashire, an 
individual (Fig. 15) was found measuring about 100 p. 
In ponds, amongst submerged vegetation in many 
parts of the country; also in marshy places amongst 
Sphagnum. 
This variety is exceedingly handsome, especially in 
the young state, when the test has a yellowish tint, 
and is quite transparent. Its dimensions vary under 
different conditions; possibly there are two varieties 
with the same kind of pitted test—one, the smaller 
and much more plentiful, measuring not more than 
40 w or 45 x; the other, represented by the Park 
Bridge example (Pl. XV, fig. 15), about twice the size, 
and with a much larger number of depressions. West 
records examples from Yorkshire of which the test, in 
some cases, was as much as 240 » in diameter. Penard 
figures examples from the Jura with the orifice of the 
test (laterally viewed) directed downwards, instead of 
—as it occurs normally in the Arcelle—upwards, facing 
the dome. These measured 80-90 » in diameter; the 
dome, 50-60 w in height. 
2. Arcella discoides Ehrenberg. 
(Plate XV, figs. 4-6, and fig. 20 in text.) 
Arcella discoidea EnRenBerG in Ber. K, Akad. Wiss. Berlin, 
1843, p. 139. 
Arcella discotdes Hurenpere in Abth. K. Akad. Wiss. Berlin, 
1871 (1872), p. 259, t. iii, f. 1; Lewy Freshw. Rhiz. N. 
Amer, (1879), p. 173, t. xxviii, ff. 14-88, and in Proc. 
Acad. Nat. Sci. Phil. 1879, p. 162; Corrapo in Boll. 
