120 BRITISH FRESHWATER RHIZOPODA. 



G. S. WEST in Journ. Linn. Soc., Zool. XXVIII (1901), 



p. 314; op, cit. XIX (1903), p. 109; and in Ann. Scott. 



Nat. Hist. 1905, pp. 89, 91 ; DANGEARD in Compt. Rend. 



1903, p. 769, and in Le Botan. IX, 1 (1903), p. 25 ; PEARL 



& DUNBAR in Biometrica, II (1903), p. 321, f. 2 R. 

 Arcella viridis PERTY in Mitth. naturf. Ges. Bern, 1849, 



pp. 160, 167, and Kenntn. kleinst. Lebensf. (1852), 



p. 186; MAGGI in Atti Soc. Ital. XXI (1878), p. 314, etc. 

 Arcella hemisphserica PERTY in Kenntn. kleinst. Lebensf. 



(1852), p. 186, t. ix, f. 5 ; PENARD in Mem. Soc. Phys. 



Geneve, XXXI, no. 2 (1890), p. 153, t. v, ff. 93-5, aud 



Faune Rhiz. Leman (1902), p. 400, ff. 

 Arcellina vulgaris CARTER in Ann. Nat. Hist. (2) XVIII 



(1856), p. 247, t. vii, f. 79. 

 Arcella patens CLAPAREDE & LACHMANN Etudes Inf. etRhiz. 2 



(1859), p. 446, t. xxii, f. 7 ; CARTER in Ann. Nat. Hist. (3) 



XIII (1864), p. 31, t. ii, f. 15. 

 Difflugia arcella WALLICH in Ann. Nat. Hist. (3) 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 



17 18 19 



FIGS. 17 and 18. Arcella vvlgaris ; common forms of test (side 



view), x 250. 

 FIG. 19. A. vulgaris, var. gibboso, ; 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 chlorophyl particles incepted as food ; 

 not completely filling the cavity, but forming attach- 

 ments by means of divergent threads; the pseudopodia 



