58 CONNECTICUT GEOL. AND NAT. HIST. SURVEY. [Bull. 



Netrium Nag. Cylindrical, straight, or fusiform cells, 

 without constriction. Chloroplasts two (or four) in each cell, 

 each with longitudinal serrate ridges. 



N. interraptum (Breb.) Lutkem, Fig. 281. 



N. Digitus (Ehrb.) Itz. and Roth., Fig. 282. 



SUB-FAMILY II. PLACODERM^E. 



Cell wall mostly constricted, with a differentiated outer 

 layer. Cell division follows a fixed type, with interpolation of 

 the younger halves between the old ones. The younger por- 

 tions of the cell wall are joined to the older portions by an 

 oblique surface. 



Key to Genera. 



1. After division the cells remain free and solitary. ... 2 

 After division the cells remain attached to form 



colonies 14 



2. Cells more or less constricted at the middle 5 



Cells not constricted 3 



3. Cells of moderate length, straight, cylindrical. . .Peniam* 

 Cells elongate, generally curved and attenuated 4 



4. Cells almost cylindrical, scarcely attenuated ; chloro- 



plast single, without apical moving granules .... Roya* 

 Cells strongly attenuated towards each extremity; 

 two chloroplasts in each cell, with apical moving 

 granules Closterium* 



5. Cells elongated and cylindrical, constriction slight. . 6 

 Cells relatively short ; deeply constricted 10 



6. Apices of cells truncate or rounded, entire 7 



Apices of cells cleft, incision open or narrow 8 



7. Base of semi-cells plicate Docidium* 



Base of semi-cells plane Plenrotcenium* 



8. Cell wall adorned with rings of furcate processes. . 



Triploceras 

 Cell wall plane 9 



9. Apical incision widely open, each apical angle 



furnished with a spine Ichthyocercus 



Apical incision narrow Tetmemorus 



10. Cells compressed (at right angles to the plane of the 



