136 BRITISH FRESHWATER RHIZOPODA. 
mouth, are characters which justify the separation of 
this form under a varietal name. In our experience it 
most frequently occurs in wet Sphagnum. The variety 
is further distinguished from the type by its being 
more compressed and altogether more delicate in 
appearance. 
With regard to C. aculeata and its varieties, it must 
be confessed there is not a little ambiguity. Forms 
are met with, chiefly in Sphagnum, which differ widely 
not only from the type, but also from each other, both 
in size and contour. Some are scarcely larger than 
average examples of Difflugia constricta, and differ so 
little from that species as to be almost indistinguish- 
able; there is, in fact, a gradation of forms connecting 
the two species (C. aculeata and D. constricta), so 
numerous that it would be a difficult and not very 
profitable task to single them out for specific mention. 
Var. ecornis (Ehrenb.) Leidy. (Plate XVI, fig. 16.) 
Arcella ecornis Wurenzerc in Abh. K. Akad. Wiss. Berlin, 
1841 (1843), p. 368, t. i, f. 9; t. ili, f. 46; in Ber. K. 
Akad. Wiss. Berlin, 1845, p. 307; op. cit. 1848, pp. 215, 
226; and op. cit. 1853, p. 178; Prircuarp Hist. Infus. 
new ed. (1852), p. 212, and ed. 4 (1861), p. 555. 
Arcella diadema (pars) Exrensrre in Abh. K. Akad. Wiss. 
Berlin, 1871 (1872), p. 259, t. iii, f. 8. 
Centropyxis ecornis Lerpy in Proc, Acad. Nat. Sci. Phil. 
1879, p. 163; TarAnex in Sitz.-ber. K. Bohm. Ges. Wiss. 
1881 (1882), p. 229; Vespovsxy Thier. Org, Brunn. Prag 
(1882), pp. 32, 48, t. iii, ff. 1-7; Frstpz in Proc. Acad. 
Nat. Sci. Phil. 1887, p.122;- Lorp in Trans. Manch. Mier. 
Soc. 1891 (1892), p. 57; Fruwzen Mikr. Fauna Argent. I, 
Prot. 4 (1897), p. 146, t. ix, ff. 18-15, in Bibl. Zool. IV. 
Centropywis aculeata var. ecornis Lyipy Freshw. Rhiz. N. 
Amer. (1879), p. 181, t. xxx, ff. 20-84; t. xxxi, ff. 88, 
34; t. xxxil, ff. 35, 86; Punarp in Mém. Soc. Phys. 
Geneve, XXX, no. 2 (1890), p. 150, t. v, ff. 45-48; G.S. 
West in Journ, Linn. Soc., Zool. XXVIII (1901), p. 317. 
Test of the same general structure as the type, but 
usually smaller; spineless. 
