18 BRITISH FRESHWATER RHIZOPODA. 
Pysxidieula Enrenserc in Abh. Akad. Berlin, 1835 (1836), 
p. 173; Infus. (1838), p. 165 (genus first defined by him). 
Cyclotella Kuntzine Bacill. oder Diatom. (1844), p. 50. 
Arcella (pars) Carvrr in Ann. Nat. Hist. (3) XIII (1864), 
p. 61. 
‘ 
Test discoid or patelliform, more or less rigid, 
punctate in appearance, transparent ; the oral aperture 
circular, large, being either the full diameter of the 
-test or somewhat contracted by an inward extension 
of it; plasma and pseudopodia similar to those of the 
genus Arcelli; nucleus usually single; contractile 
vesicles one or more. 
Only a few species are included in this genus. 
P. patens Clap. & Lachm. is discoid and has a short 
funnel-shaped rim attached to the inner side of the 
inverted edge of the test which is about 40 p in 
diameter. Three species have been recorded from 
the British Isles. 
SYNOPSIS OF THE British SPECIES. 
Test very small, discoid, circular, with inverted aperture. 
(1) P. operculata. 
Test rather larger, discoid, with bifurcated margin. 
(2) P. invisitata. 
Test much larger, patelliform, with external border. 
(3) P. cymbalum. 
1. Pyxidicula operculata (Agardh) Ehrenberg. 
(Figs. 164-166.) 
Frustulia operculata 
AGARDH in Flora (Bot. Zeit.), X, 2 (1827), p. 627, 
Cymbella operculata 
AGARDH Consp. crit. Diatom. (1830), p. 11. 
Frustulia (Cyclotella) operculata 
Kverzine in Linnea, 1833, pp. 5385-537, pl. xiii, f. 1; Synops. Diatom. 
(1834), p. 7, pl. vii, £. i. 
Gallionella ¢ (Pyxidicula) operculata 
EHRENBERG in Abh. Akad. Berlin, 1833 (1834), p. 265. 
Pyxidicula oe 
ExRENBERG in Abh. Akad. Berlin, 1835 (1836), p. 173; Infus. 
p. 165, pl. x, f. 1; Mikrogeol. (1854), ert : 3 46 a, b. es 
