132 BRITISH FRESHWATER RHIZOPODA. 
Test discoid, circular, or oval, with an excentric 
orifice ; obtusely rounded posteriorly, where it is also 
thickest, thinning off towards the opposite edge, near 
which the mouth is situated. Mouth circular or oval, 
entire or with a sinuous or lobate border; the test 
furnished with a variable number of conical or curved, 
sometimes bifid, spimes, its substance chitinous, ex- 
hibiting various shades of brown, or almost colourless; 
sometimes covered with extraneous elements (sand- 
grains, etc.) and without spines. The protoplasm 
colourless ; pseudopodia digitate. 
Leidy considered Centropyzis to be distinctly 
separated from Aicella, while on the other hand it 
appears closely related with Diflugia, through D. 
coustricta. The relation is almost too close to justify 
the retention of the latter among the Diffluqie. 
The priority of Centropyais over LHechinopyvris is 
doubtful, but Stein’s paper was vead in 1857, and his 
name (Cvutropyvis) is the more appropriate one. 
1. Centropyxis aculeata (Ehrenb.) Stein. 
(Plate XVI, figs. 10-14.) 
Arcella aciuleata Eurenpere in Abh. K. Akad. Wiss. Berlin, 
1830 (1832), p. 40; op. cit. 1831 (1882), p. 91; op. cit. 
1841 (1842), p. 368, t. iii, f. 5; Infus. (1838), p. 183, 
t. ix, f. vi; in Ber. K. Akad. Wiss. Berlin, 1847, p. 477 ; 
and op, cit. 1853, p. 255; Prrrenarp Hist. Infus. (1842), 
p. 169, t. ii, ff. 92-94; new ed. (1852), p. 211, t. u, 
ff. 92-94; and ed. 4 (1861), p. 555; Barey in Smithson. 
Contrib. II (1851), art. 8, passim; ArcuEr in Q. J. Micr. 
Sci. VI, ns. (1866), pp. 185, 186. 
slreellina aculeata Canon in Ann. Nat. Hist. (1) LX (1842), 
pp. 362, 493. 
Difugin aeuleata Perry in Mitth. nat. Ges. Bern, 1849, 
pp. 45, 162, and Kenntn. kleinst. Lebensf. (1852), p. 186; 
Lemy in Proc. Acad. Nat. Sci. Phil. 187-4, p. 14; Mager 
in Atti Soc. Ital. XXT (1879), p. 319. 
Ceutropyris aculeata Stun in Abh. K. Bohm. Ges. Wiss. X 
(1859), Ber. p. 43; Lurmy Freshw. Rhiz. N. Amer. (1879), 
p. 180, t. xxxi, ff. 1-82; t. xxxu, ff. 29-34; in Proce, 
Acad, Nat. Sci. Phil. 1879, p. 163; and op. ef. 1880, 
