FRESHWATER RHIZOPODS AND HELIOZOA. 317 
smaller punctulations. ‘lhe structure of the shell-membrane of 
the particular forms described was thus of a somewhat different 
nature from the cancellated structure exhibited by the shells of 
Arcella. (Cfr. Pl. 29. figs. 15, 16.) 
Var. rcornis, Leidy,l. c. p. 180, t. xxx. ff. 20-34, t. xxxi. ff. 33, 
34.—Arcella ecornis, Ehrenb. Abhand. Akad. Wiss. Berlin, 1841, 
p- 368, t.i. f£. 9, t. ui. f. 46. Centropyxis levigata, Penard, in 
Mem. Soe. Phys. et d’Hist. Nat. Geneve, tom. xxxi. no. 2, 1890, 
p- 151. t. v. ff. 42-44, 49-55. 
Not so abundant as the typical spined form. Obtained it very 
abundantly on damp moss on limestone rocks, Ingleton, N. York- 
shire. Also from near Bowness, Westmoreland. 
Gen. Dirriveta, Leclere. 
18. Dirriueia constRicta, Lezdy, Freshw. Rhiz. N. Amer. 
1879, p. 120, t. xviii.—Arcella constricta, Ehrenb. Abhand. Akad. 
Wiss. Berlin, 1841, p. 410, t. iv. f. 35, t. v. f. 1. Difflugia 
marsupiformis, Wallich, in Ann. Mag. Nat. Hist. xiii. 1864, 
pp. 241, 244, t. xvi. ff. 3-5. D. platystoma, Penard, in Meém. 
Soc. Phys. et d’ Hist. Nat. Genéve, tom. xxxi. no. 2, 1890, p. 148, 
t. iv. ff. 85-37. 
A very abundant species which exhibits much variation, many 
of the forms being practically indistinguishable from Centropyzis 
aculeata var. ecornis. ‘The highest elevation at which | obtained 
it was 3000 ft. on Snowdon, N. Wales. 
19. Dirriveta pyrirormis, Perty, 1848; Kennt. kleinst. 
Lebensf. 1852, p. 187, t. ix. f.9; Leidy, Freshw. Rhiz. N. Amer. 
1879, t. x., xi., xii. ff. 1-18, ete.—D. compressa, Carter, in Ann. 
Mag. Nat. Hist. xui. 1864, p. 22, t. i. ff. 5,6. D. entochloris, 
Leidy, in Proc. Acad. Nat. Sci. Philad. 1874, p. 79; 1875, p. 307. 
D. saxicola, Penard, in Mém. Soc. Phys. et d’ Hist. Nat. Geneve, 
tom. xxxi. no. 2, 1890, p. 188, t. iii. ff. 50-52. D. avellana, 
Penard, 1. ¢. p. 144, t. iv. ff. 88-40. D. fallax, Penard, /.c. t. iv. 
ff. 41-51. D. lucida, Penard, 1. c. p. 145, t. iv. ff. 52-58. D. 
bacillifera, Penard, et var. inflata, Penard,l.c. p.46, t. iv. ff.61-71. 
D. lanceolata, Penard, l. c. p. 145, t. iv. ff. 59, 60. 
The most widely distributed species of the genus, especially in 
quiet waters. It is subject to very great variation, not only in 
outward form, but also in the nature of the materials composing 
