ARCELLA POLYPORA. 17 
basal angles; crown evenly convex; base concave ; 
aperture circular, about one-third the diameter of the 
test in width, surrounded by a ring of small circular 
markings; plasma colourless, granular; nuclei small, 
from 6 to 20 in number, each with a large central 
nucleole; contractile vesicles numerous ; pseudopodia 
digitate. 
Diameter 100-200 »; aperture 0°3 to 04 of .the 
diameter ; height one-fourth to one-third the diameter. 
Hatlitat—Sphagnum and aquatic vegetation. 
EnceirandD.—N. Yorkshire; Shropshire; Bucking- 
hamshire. 
Iretanp.—Clare Island, Mayo. 
Those Arcelle which have disc-shaped tests form a 
small group comprising four species which, except A. 
discoides, are distinguished from all other members of 
the genus by possessing more than two nuclei; A. 
discoides has also by some observers been recorded as 
multinuclear, but it is probable that in these cases a 
process of division was in progress, or perhaps A. 
polypera was under observation. The following table 
shows the approximate sizes and mutual relationship of 
the members of this group ; in all of them the height 
of the test may vary from one-third to one-fourth or 
less of the diameter. 
Diameter. Aperture. No. of nuclei. 
A. discoides Ehrenb. . 80-150 py 0°3 of the diameter . 2 normally 
A. polypera Penard . 100-2004 . 0°3-0°4 5 ; 6-20 
A, megastoma Penard 190-365 p . 0°4-0°55 9% . 40-200 
A. curvata Wailes  . 120-1354 . 0°4 35 ; 4-5 
Genus 15a. PYXIDICULA Ehrenberg, 1834. 
Frustulia (pars) Acarp# in Flora (Bot. Zeit.), X, 2 (1827), 
. 627. 
Opies (pars) Acarpu Consp. crit. Diatom. (1830), p. 11. 
Frustulia (Cyclotella) Kunrzine in Linnea, 1833, p. 535. 
Gallionella? (Pyaidicula) Enrensere in Abh. Akad. Berlin, 
1833 (1834), p. 265. 
VOL. Iv. 2 
