122 BEITISH FRESHWATER RHIZOPODA. 



Test gibbous, varying structurally from the type, 

 and from var. angulosa, 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/x; 

 sometimes much larger. At Park Bridge, Lancashire, an 

 individual (Fig. 15) was found measuring about 100 /x. 



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 p, or 45 p. ; the other, represented by the Park 

 Bridge example (PI. 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 /A 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 ArcellsB upwards, facing 

 the dome. These measured 80-90 /x in diameter ; the 

 dome, 50-60 /A in height. 



2. Arcella discoides Ehrenberg. 

 (Plate XV, figs. 4-6, and fig. 20 in text.) 



Arcella discoidea EHRENBERG in Ber. K. Akad. Wiss. Berlin, 



1843, p. 139. 

 Arcella discoides EHRENBERG in Abth. K. Akad. Wiss. Berlin, 



1871 (1872), p. 259, t. iii, f. 1 ; LEIDY Freshw. Rhiz. N. 



Amer. (1879), p. 173, t. xxviii, ff. 14-38, and in Proc. 



Acad. Nat. Sci. Phil. 1879, p. 162; CORRADO in Boll. 



