124 BRITISH FRESHWATER RHIZOPODA. 



Dimensions : Diameter variable, averaging about 

 150 ^ 



Frequent in ponds and marshes ; less common than 

 A. 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 (PI. XV, fig. 6). These are almost 

 invariably young forms. 



Penard, in ' Mem. Soc. de Phys. et d'Hist. Nat. de 

 Geneve,' 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/u, 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) ; COBRADO in Boll. 

 Scient. I, an. 2 (1880), pp. 47, 48; HITCHCOCK Synops. 

 Freshw. Rhiz. (1881), p. 26; 'TARANEK in Sitz.-ber K. 

 Bohm. Ges. Wiss. 1881 (1882), p. 225; BLOCHMANN Mikr. 

 Thierw. Siisswass (1886), p. 12, and ed. 2 (1895), p. 15 ; 

 PERRY in Proc. Amer. Soc. Micr. XII (1891), p. 95 ; LORD 

 in Trans. Manch. Micr. Soc. 1891 (1892), p. 57 ; G. S. 

 WEST in Journ. Linn. Soc., Zool. XXVIII (1901), p. 315; 

 PENARD Faune Rhiz. Leman (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 



