l6 DAVENPORT ACADEMY OF SCIENCES. 



Those possessing the three-lobed mouth are the most common 

 forms of this state. 



DiFFLUGiA CORONA Wallich. 



Shell spheroidal, composed of particles of quartz sand, the fun- 

 dus usually being spined. Mouth terminal, circular, with a 

 notched border. 



Length, 175 microns. (Fig. 16, PI. III.) 



Diffliigia corona is one of the larger species of the genus. The 

 distribution is general, being found in great abundance in man}' 

 localities in the state. Lake Okoboji, during the summer of 

 1905, furnished myriads of this species. Commonly Difflugia 

 corona may be found resting with its mouth downward while the 

 cylindrical pseudopodia protrude in a radiating manner. 



The small tooth-like processes bordering the mouth may vary 

 in number but are usually more than ten. The number of spines 

 also varies, but seldom less than four or five to an individual have 

 been observed. Frequently found among algae. 



DlFFI^UGIA SPIRALIS Ehr. 



Shell retort-shaped with an evenly rounded fundus, laterally 

 compressed. A short neck usually produced. 



Length, 125 microns. (Fig. 18, PI. IIL) 



This species is a rare one in this state, having been found but 

 a few times and only in Johnson county. 



The spiral appearance of the shell is due to an interior parti- 

 tion arising from the concavity of the side of the body and extend- 

 ing to an upward curve toward the other side. This partition 

 does not entirely separate the cavity but serves as a wall, behind 

 which the protoplasm may retract. When viewed through the 

 more or less transparent shell the partition resembles a dark line. 



Found in pond water among aquatic plants. 



ARCELLA Ehrenberg. 



Body secreting a shell of chitinoid material, semi-spheroidal in 

 shape when viewed laterally, usually circular when viewed dors- 

 ally or ventrally; mouth central; pseudopodia digitate or broadly 

 lobate, never branched. Nuclei and contractile vesicles usually 

 more than one. 



