IOWA ACADEMY OF SCIENCE 
75 
Cells elongate, generally curved; symmetrical in one longitudinal plane 
only. Cell wall commonly with pores. Points of division regularly 
placed in the middle region of the cell II. Closterieae 
Cells exhibit a great variety of form, and the cell wall consists of two 
thin, firm layers with pores. There is no periodical growth, the cell be- 
coming adult immediately after division by the mature growth of the 
semi-cell III. Cosmarieae 
Tribe I. 
Penieae. 
1. Penium. 
132. P. closterioides Ralfs. 
East Okoboji, in stagnant pool by water’s edge. Buchanan. 
Ames. Roadside pool. Buchanan. 
Tribe II. 
Closterieae. 
1. Closterium. 
KEY TO THE SPECIES. 
133. 
13h. 
135. 
136. 
137. 
138 . 
Cells more or less cylindrical, slightly bent, ends scarcely or not at all 
tapering. Diameter, 50-55 u C. lanceolatum 
Cells slightly bent, back or dorsum slightly convex, opposite almost 
straight, distinctly tapering. 
Diameter cells 35-62 u C. acerosum 
Diameter cells 80-110 u C. lunula 
Cells more or less curved, dorsal and ventral margins both convex, 
arched in the same direction, ends sensibly tapering, lunate. 
Diameter cell 14 u C. jenneri 
Diameter cell 18-25 u C. dianae 
Diameter cell 25-28 u C. acuminatum 
Cells falcate incurved, ventral margin swollen, ends sensibly tapering, 
36-55 u in diameter C. moniliferum 
C. lanceolatum Kuetz. 
Iowa City. Hobby. 
C. acerosum Ehrenb. 
Eagle Grove. Jenning’s Pond. Buchanan. 
Eagle Grove. Hewitt’s Pond. Buchanan. 
C. lunula Nitzsch. 
Iowa City. Hobby. 
C. jenneri Ralfs. 
Eagle Grove. Jenning’s Pond. Buchanan. 
Eagle Grove. Slough. Buchanan. 
C. dianae Ehrenb. 
Eagle Grove. Slough, amid Utricularia. Buchanan. 
C. acuminatum Kuetz. 
Eagle Grove. Slough, amid decaying rushes. Buchanan. 
