ARCELLA DISCOIDES. 123 
Scient. I, an. 2 (1880), p.47; Hircucock Synops. Freshw. 
Rhiz. (1881), p. 26; Waurreteeee in Proc. Linn. Soc. N.S. 
Wales, (2) I (1886), p. 501; Fretpz in Proc. Acad. Nat. 
Sci. Phil. 1887, p. 122; Harvey in Amer. Nat. XXII 
(1888), p. 73; PENaro i in Mén, Soc. Phys. Genéve, XX XI, 
no. 2 (1890), ’p. 153, t. v, ff. 70-74; in Rev. Suisse Zool. 
VII, 1 (1899), pp. 104, 107; and Faune Rhiz. Léman 
(1902), p. 402, ff.; Perry in Proc. Amer. Soc. Mier. 
XII (1891), p. 95; Cash in Trans. Manch. Micr. Soc. 
1891 (1892), p. 51; Lorp in Trans. Manch. Micr. Soc. 
1891 (1892), p. 57; Luvanper in Acta Soc. Fauna Fenn. 
XIT (1894), no. 2, p. 13; Dapay Micr. Siisswass. Ceylon 
(1898), pp. 5, 9; G. 8S. 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, 92. 
Arcella vulgaris var. discoides Isstz in Atti Acc. Torino, 
XXXVI (1901), p. 64. 
Arcella peristicta Hurenpera Microgeol. (1854), p. 331; 
and in Abth. K. Acad. Wiss. Berlin, 1871 (1872), p. 260, 
t. ii, ff. 11, 18. 
Arcella discoides (Ehrenb.) ? Lurpy in Proc. Acad. Nat. Sci. 
Phil. 1876, p. 56. 
Test discoid, circular in dorsal or ventral view, ex- 
panded, less convexly rounded at the basal angles 
than A. vulgaris, and of much greater breadth; also 
more transparent, the surface smooth, faintly punctated ; 
the mouth central. In lateral view plano-convex, 
the breadth from the base to the apex of the dome 
measuring only about a quarter or third of the 
diameter; the crown convex, sloping evenly down to 
the expanded and but slightly rounded basal border. 
Protoplasm, etc., as in the allied species. 
Fia. 20. Arcella discoides; ordinary pond-form (face view). x 200. 
