AKCELLA 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 p ; aperture 0'3 to 0'4 of the 

 diameter ; height one-fourth to one-third the diameter. 



Habitat. Sphagnum and aquatic vegetation. 



ENGLAND. N. Yorkshire; Shropshire; Bucking- 

 hamshire. 



IRELAND. Clare Island, Mayo. 



Those Arcellae 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 

 inultinuclear, but it is probable that in these cases a 

 process of division was in progress, or perhaps A. 

 poli/pera 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 ft . 0'3 of the diameter . 2 normally 

 A. polypera Penard . 100-200 /i . 0'3-0'4 . 6-20 



A. megastoma Penard 190-365 ft . 0'4-0'55 . 40-200 



A. curvata Wailes . 120-135 p . 0'4 .4-5 



Genus 15 a. PYXIDICULA Ehrenberg, 1834. 



Frustulia (pars) AGARDH in Flora (Bot. Zeit.), X, 2 (1827), 



p. 627. 



Cymbella (pars) AGARDH Consp. crit. Diatom. (1830), p. 11. 

 Frustulia (Cyclotella) KUETZING in Linnea, 1833, p. 535. 

 Gallionella ? (Pyxidicula) EHRENBERG in Abh. Akad. Berlin, 



1833 (1834), p. 265. 

 VOL. IV. 2 



