AECELLA DISCOIDES. 123 



Scient. I, an. 2 (1880), p. 47; HITCHCOCK Synops. Fresh vv. 



Rhiz. (1881), p. 26; WHITELEGGE in Proc. Linn. Soc.N. S. 



Wales, (2) 1 (1886), p. 501 ; FIELDE in Proc. Acad. Nat. 



Sci. Phil. 1887, p. 122; HAKVEY in Amer. Nat. XXII 



(1888), p. 73 ; PENARD in Mem. Soc. Phys. Geneve, XXXI, 



no. 2 (1890), p. 153, t. v, ff. 70-74; in Rev. Suisse Zool. 



VII, 1 (1899), pp. 104, 107; and Faune Rhiz. Lemaii 



(1902), p. 402, ff.; PERRY in Proc. Amer. Soc. Micr. 



XII (1891), p. 95; CASH in Trans. Manch. Micr. Soc. 



1891 (1892), p. 51 ; LORD in Trans. Manch. Micr. Soc. 



1891 (1892), p. 57; LEVANDER in Acta Soc. Fauna Fenn. 



XII (1894), no. 2, p. 13 ; DADAY Micr. Siisswass. Ceylon 



(1898), pp. 5, 9; Gr. 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, 92. 

 Arcella vulgaris var. discoides ISSEL in Atti Ace. Torino, 



XXXVI (1901), p. 64. 

 Arcella peristicta EHRENBERG Microgeol. (1854), p. 331 ; 



and in Abth. K. Acad. Wiss. Berlin, 1871 (1872), -p. 260, 



t, iii, ff. 11, 13. 

 Arcella discoides (Ehrenb.) ? LEIDY in Proc. Acad. Nat. Sci. 



Phil. 1876, p. 56. 



Test discoid, circular in dorsal or ventral view, ex- 

 panded, less con vex ly 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. 



FIG. 20. Arcella discoides ; ordinary pond-forin (face view), x 200. 



