l8 DAVENPORT ACADEMY OF SCIENCES. 



ual does not complete!}' fill the shell but is attached to the inner 

 surface by means of minute threads. By the liberation of carbon 

 dioxide within the shell, at any point, the organism has the power 

 to change its position in the water at will by changing its specific 

 gravity; thus the species may raise itself in the water or turn on 

 its side. At least two nuclei are usually present, opposite in posi- 

 tion. Contractile vesicles may be numerous. 



Arcella discoides Ehr. 



Shell comparatively large, flattened, disc-like. Mouth circular 

 and of large diameter. 



Diameter of shell, 125 microns. (Figs. 25-26, PI. IV.) 



This species differs from the preceding one in three particulars: 

 the diameter of the shell, the height of the dome, and the diame- 

 ter of the mouth. 



The height of Arcella discoides is frequently less than one-fourth 

 its diameter and the diameter of the mouth often equals one-half 

 the diameter of the shell. 



Figure 26, Plate IV, represents a typical form of this species 

 from a lateral point of view. Fig. 25 is a dorsal view showing 

 extended pseudopodia. 



The surface of Arcella discoides is usually marked by punctae, 

 but no shallow concavities have been observed as in the case of 

 Arcella vulgaris. 



Although widely distributed the individuals are usually iso- 

 lated, not appearing in aggregate quantities. A few specimens 

 have been observed in many localities in this state. Found in 

 pond water among aquatic plants and often associated with Dif- 

 flugia . 



CENTROPYXIS Stein. 



Shell composed of chitinous material with the addition of sand 

 grains, diatom shells, etc.; ovoid in shape, with the mouth and 

 fundus eccentric in opposite directions. Spines usually present. 



Centropvxis aculeata Stein. 



Having the characters of the genus. 



Eength of shell, 150 microns. 



Diameter of mouth, 60 microns. (Figs. 27-28, PI. IV.) 



