GEISTUS AECELLA— AECELLA DISCOIDES. 173 



The shell of Arcella vulgaris ranges from ^ th to ~th of an inch in 

 breadth, —th. to ^th of an inch in height, with the mouth ^th to if,h of 

 an inch in breadth, and elevated ^th to ^th of an inch. 



ARCELLA DISCOIDES. 



Plate XXVIII, figs. 14-38. 



Arcella disunite). Ehrenberg : Monatsb. Ak. Wis. Berlin, 1843, 139 ; Abh. Ak. Wis. Berlin, 1871, 259, 



Taf. iii, Fig. 1.— Leidy : Proc. Ac. Nat. Sc. 1876, 56. 

 A. Homococldtimys disc/rides. Elirenberg: Ab. Ak. Wis. 1871, 244. 



Arcella pcristicta. Ehrenberg : Microgeologie, 1854, 331 ; Ab. Ak. Wis. 1871, 260, Taf. iii, Fig. 11, 12. 

 A. Heierocosmia poristicta. Ehrenberg: Ab. Ak. Wis. 1871, 245. 



Shell mostly circular, shield-shaped, usually with the height from one 

 fourth to one third of the breadth ; dome low, evenly convex to the rounded 

 or slightly expanded and rounded basal border ; base and mouth, and like- 

 wise color of the shell, as in Arcella vulgaris. Sarcode as in the latter. 



Size. — Breadth from 0.072 mm. to 0.264 mm. ; height from 0.02 mm. to 

 0.08 mm ; width 0.02 mm. to 0.08 mm. ; elevation of mouth 0.108 mm. to 

 0.02 mm. 



Locality. — Everywhere in association with Arcella vulgaris. Pennsyl- 

 vania, New Jersey, Florida, Alabama, and Fort Bridger and Uinta Mount- 

 ains, Wyoming Territory. 



Arcella discoides (pi. XXVIII, figs. 14-38) I view as the variety 

 of A. vulgaris in which the shell presents a greater proportionate reduction 

 in height compared with the breadth; but the one graduates into the other. 

 Usually with the height from a little less than a fourth to little more than a 

 third of the breadth, the shell appears shield-shaped, with an even convex 

 surface, neither faceted nor pitted. The base has the same character as in 

 A. vulgaris. The mouth varies greatly in its size in proportion to that of 

 the shell, ranging from one fourth to one half the corresponding diameter 

 of the latter. 



From the comparative shallowness of depth of the shell, especially in 

 the colorless condition, Arcella discoides is especially well adapted for the 

 investigation of its contents. 



The description and figure of A. discoides of Ehrenberg appear to 

 apply to a large-mouthed colorless individual. 



The sarcode mass usually forms a disk, convex above, with rounded 

 or obtusely angular border, and impressed below, where it rests on the 



