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 G. 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 (G. 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 EHEENBERG in Ahli. K. Akad. Wiss. Berlin, 

 1841 (1843), p. 368, t, i, f. 9 ; t. iii, f. 46; in Ber. K. 

 Akad. Wiss. Berlin, 1845, p. 307 ; op. cit. 1848, pp. 215, 

 226; and op. cit. 1853, p. 178; PRITCHARD Hist. Infus. 

 new ed. (1852), p. 212, and ed. 4 (1861), p. 555. 



Arcella diadema (pars) EHRENBERG in Ahh. K. Akad. Wiss. 

 Berlin, 1871 (1872), p. 259, t. iii, f. 8. 



Centropyxis ecornis LEIDY in Proc. Acad. Nat. Sci. Phil. 

 1879, p. 163; TARANEK in Sitz.-ber. K. Bohm. Ges. Wiss. 

 1881 (1882), p. 229 ; VEJDOVSKY Thier. Org. Brunn. Prag 

 (1882), pp. 32, 43, t. iii, ff. 1-7 ; FIELDE in Proc. Acad. 

 Nat. Sci. Phil. 1887, p.122; LORD in Trans. Manch. Micr. 

 Soc. 1891 (1892), p. 57; FRENZEL Mikr. Fauna Argent. I, 

 Prot. 4 (1897), p. 146, t. ix, ff. 13-] 5, in Bibl. Zool. IV. 



Centropyxis aculeata var. ecornis LEIDY Freshw. Rhiz. N. 

 Amer. (1879), p. 181, t. xxx, ff. 20-34; t. xxxi, ff. 33, 

 34 ; t. xxxii, ff. 35, 36 ; PENARD in Mem. Soc. Pliys. 

 Geneve, XXXI, 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. 



