120 BRITISH FRESHWATER RHIZOPODA. 
G.S. Wes in Journ. Linn. Soc., Zool. XXVIIT (1901), 
p. 314; op. cit. XIX (1903), p. 109; and in Ann. Scott. 
Nat. Hist. 1905, pp. 89, 91; Danezarp in Compt. Rekd. 
1903, p. 769, and in Le Botan. IX, 1 (1903), p. 25; Prarn 
& Dunzpar in Biometrica, II (1903), p. 321, f. 2 R.° 
Arcella viridis Prrty in Mitth. naturf. Ges. Bern, 1849, 
pp. 160, 167, and Kenntn. kleinst. Lebensf. (1852) 
p. 186; Magali in Atti Soc. Ital. XXI (1878), p. 314, etc. 
Arcella hemispherica Perry in Kenntn. kleinst. Lebensf. 
(1852), p. 186, t.ix, f. 5; Pewarp in Mém. Soc. Phys. 
Genéve, XXXI, no. 2 (1890), p. 153, t. v, ff. 93-5, and 
Faune Rhiz. Léman (1902), p. 400, ff. 
Arcellina vulgaris Carter in Ann, Nat. Hist. (2) XVIII 
(1856), p. 247, t. vii, £. 79. 
Arcella patens CLararipe & Lacumann Ktudes Inf. et Rhiz. 2 
(1859), p. 446, t. xxii, f. 7; Carrer in Ann. Nat. Hist. (3) 
XIII (1864), p. 31, t. ii, £. 15. 
Difflugia arcella Watuicu in Ann. Nat. Hist. (8) XIII (1864), 
p. 245, t. xvi, ff. 34-38. 
Test in dorsal or ventral aspect discoid, circular, 
with a central orifice, of which the margin is usually 
plain, but in some examples faintly crenulated ; with a 
2 
Fies. 17 and 18.—Arcella vulgaris; common forms of test (side 
view), x 250. 
Fig. 19.—A. vulgaris, var. gibbosa; ordinary small pond form (side 
view), x 300, 
more or less distinctly but finely punctated surface. 
The mouth is concentric with the outer periphery of 
the test. In lateral view the outline is nearly or quite 
hemispherical, with convex borders; the height of the 
crown one half the breadth of the test—the dome 
evenly convex, and smooth, except in the varieties 
referred to. The protoplasm is colourless except for 
the presence of chlorophy] particles incepted as food ; 
not completely fillmg the cavity, but forming attach- 
ments by means of divergent threads; the pseudopodia 
