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, spines, 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 Centropyxis to be distinctly 

 separated from Arcella, while on the other hand it 

 appears closely related with Difflugia, through D. 

 con strict a. The relation is almost too close to justify 

 the retention of the latter among the Difflugise. 



The priority of Gentropy&is over Echinopyxis is 

 doubtful, but Stein's paper was read in 1857, and his 

 name (Gentropijms) is the more appropriate one. 



1. Centropyxis aculeata (Ehrenb.) Stein. 

 (Plate XVI, figs. 10-14.) 



Arcella aculeata EHRKNBERG in Abh. K. Akad. Wiss. Berlin, 

 1830 (1832), p. 40; op. cit. 1831 (1832), p. 91; op. cit. 

 1841 (1842), p. 368, t. in, f. 5; Infus. (1838), p. 133, 

 t. ix, f. vi ; in Ber. K. Akad. Wiss. Berlin, 1847, p. 477 ; 

 and op. cit. 1853, p. 255; PRITCHARD Hist. Infus. (1842), 

 p. J69, t. ii, ff. 92-94; new ed. (1852), p. 211, t. ii, 

 ff. 92-94 ; and ed. 4 (1861), p. 555 ; BAILEY in Smithson. 

 Contrib. II (1851), art. 8, passim; ARCHER in Q. J. Micr. 

 Sci. VI, n.s. (1866), pp. 185, 186. 



Arcellina aculeata CANTON in Ann. Nat. Hist. (1) IX (1842), 

 pp. 362, 493. 



Difflugia aculeata PERTY in Mitth. nat. Ges. Bern, 1849, 

 pp. 45, 162, and Kenntn. kleinst. Lebensf. (1852), p. 186; 

 LEIDY in Proc. Acad. Nat. Sci. Phil. 1874, p. 14; MAGGI 

 in Atti Soc. Ital. XXI (1879), p. 319. 



Centropyxis aculeata STEIN in Abh. K. Bohm. Gres. Wiss. X 

 (1859), Ber. p. 43 ; LEIDY Freshw. Rhiz. N. Ainer. (1879), 

 p. 180, t. xxxi, ff. 1-32 ; t. xxxii, ff. 29-34 ; in Proc. 

 Acad. Nat. Sci. Phil. 1879, p. 163 ; and op. cit. 1880, 



