'18 BRITISH FRESHWATER RHIZOPODA. 



Pyxidicula EHRENBERG in Abh. Akad. Berlin, 1835 (1836), 

 p. 173 ; Infus. (1838), p. 165 (genus first defined by him). 



Cyclotella KUETZING Bacill. oder Diatom. (1844), p. 50. 



Arcella (pars) CARTER 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 Arcella ; 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. 



i 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 



KUETZING in Linnea, 1833, pp. 535-537, pi. xiii, f. 1; Synops. Diatom. 



(1834), p. 7, pi. vii, f. i. 

 Gallionella > (Pyxidicula) operculata 



EHRENBERG in Abh. Akad. Berlin, 1833 (1834), p. 265. 

 Pyxidicula operculata 



EHRENBERG in Abh. Akad. Berlin, 1835 (1836), p. 173; Infus. (1838), 

 p. 165, pi. x, f. 1 ; Mikrogeol. (1854), pi. xvi, i, f. 46 a, b. 



