. The British freshwater Rhizopoda and Heliozoa. I (1894), no. 2, p. 13; Dadat Micr. Siisswass. Cevlon (1898), pp. 5, 9; G. S. West in Journ. Linn. Soc, Zool. XXVIII (1901), p. 314; op. cit. XXIX (1903), p. 109; and in Ann. Scott. Nat. Hist. 1905, pp. 89, vulgaris var. discoides Issel in Atti Ace. Torino, XXXVI (1901), p. peristicta Bhrenbeeg Microgeol. (1854), p. 331; and in Abth. K. Acad. Wiss. Berlin, 1871 (1872), p. 260, t. iii, &. 11, discoides (Ehrenb.) ? Leidy in Proc. Acad. Nat. Sci. Phil. 1876, p. 56. Test discoid, circular in dorsal or ventral yiew, e
. The British freshwater Rhizopoda and Heliozoa. I (1894), no. 2, p. 13; Dadat Micr. Siisswass. Cevlon (1898), pp. 5, 9; G. S. West in Journ. Linn. Soc, Zool. XXVIII (1901), p. 314; op. cit. XXIX (1903), p. 109; and in Ann. Scott. Nat. Hist. 1905, pp. 89, vulgaris var. discoides Issel in Atti Ace. Torino, XXXVI (1901), p. peristicta Bhrenbeeg Microgeol. (1854), p. 331; and in Abth. K. Acad. Wiss. Berlin, 1871 (1872), p. 260, t. iii, &. 11, discoides (Ehrenb.) ? Leidy in Proc. Acad. Nat. Sci. Phil. 1876, p. 56. Test discoid, circular in dorsal or ventral yiew, ex-panded, less convexly rounded at tlie basal anglesthan A. vulgaris, and of much greater breadth; alsomore transparent,the surface smooth,faintly punctated;the mouth central. In lateral view plano-convex,the breadth from the base to the apex of the domemeasuring only about a quarter or third of thediameter; the crown convex, sloping evenly down tothe expanded and but slightly rounded basal , etc., as in the allied Fia. 20. Arcella discoides; ordinary pond-form (face view), x 200. 124 BRITISH FRESHWATER RHIZOPODA. Dimensions: Diameter variable, averaging about150/A. Frequent in ponds and marstes; less common thanA. vulgaris. Arcella discoides, though Leidy thought it mightprobably be only a variety of A. vulgaris, is readilydistinguished by its greater delicacy and transparency,and by its much shallower and more widely-expandedtest. Individuals sometimes occur whose tests areeither immature, or ill-developed, or have sustainedsome injury (PI. XV, fig. 6). These are almostinvariably young forms. Penard, in Mem. Soc. de Phys. et dHist. Nat. deGreneve, 1890, records, under the name of A. polypora,a form with the mouth distinctly everted, but in otherrespects, except as regards size, hardly differing fromA. discnides. It is smaller, measuring 80-120 /a indiameter and 10-15/a in height. 3. Arcella mitrata Leidy. (Plate XV, figs. 9 and 10.) Arcella mitrata
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