124 BRITISH FRESHWATER RHIZOPODA. 
Dimensions: Diameter variable, averaging about 
150 p. 
Frequent in ponds and marshes; less common than 
al. vulgaris. 
Arcella discoides, though Leidy thought it might 
probably be only a variety of A. vulgaris, is readily 
distinguished by its greater delicacy and transparency, 
and by its much shallower and more widely-expanded 
test. Individuals sometimes occur whose tests are 
either immature, or ill-developed, or have sustained 
some injury (Pl. XV, fig. 6). These are almost 
invariably young forms. 
Penard, in ‘Mem. Soc. de Phys. et d’Hist. Nat. de 
Genéve,’ 1890, records, under the name of A. polypora, 
a form with the mouth distinctly everted, but in other 
respects, except as regards size, hardly differing from 
A, discoides. It is smaller, measuring 80-120 p in 
diameter and 10-15 p in height. 
3. Arcella mitrata Leidy. 
(Plate XV, figs. 9 and 10.) 
Arcella mitrata Leidy in Proc. Acad. Nat. Sci. Phil. 1876, 
p. 76; op. cit. 1879, p. 162; and Freshw. Rhiz. N. Amer. 
(1879), p. 175, t. xxix (? excl. fig. 52) ; Corrapo in Boll. 
Scient. I, an. 2 (1880), pp. 47, 48; Hircncock Synops. 
Freshw. Rhiz. (1881), p. 26; TarAwex in Sitz.-ber K. 
Bohm. Ges. Wiss. 1881 (1882), p. 225; Brocumann Mikr. 
Thierw. Siisswass (1886), p. 12, and ed. 2 (1895), p. 15; 
Perry in Proc. Amer. Soc. Micr. XII (1891), p. 95; Lorp 
in Trans. Manch. Micr. Soc. 1891 (1892), p. 57; G.S. 
West in Journ. Linn. Soc., Zool. XXVIIT (1901), p. 315 ; 
Penarp Faune Rhiz. Léman (1902), p. 576, f. 8. 
Test mitriform or balloon-shaped, ob-pyriform or 
polyhedral, higher than the breadth of the base, 
widest at or near the middle, more or less contracted 
or sloping inwardly towards the base; the dome 
mostly inflated, its summit and sides evenly rounded, 
or depressed into broad angular facets bounded by 
